metano disuelto, difusividad

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CM http://dx.doi.org/10.7773/cm.v39i2.2232

Dissolved methane in the sills region of the Gulf of California

Metano disuelto en la regin de umbrales del golfo de California

Jos Vinicio Macas-Zamora1*, Karel Castro-Morales1,4, Roger Allen Burke2,Manuel Lpez-Mariscal3

1 Instituto de Investigaciones Oceanolgicas, Universidad Autnoma de Baja California, Carretera Tijuana-Ensenada No. 3917, Fraccionamiento Playitas, Ensenada, CP 22860, Baja California, Mxico.

2 US Environmental Protection Agency, National Exposure Research Laboratory, 960 College Station Road, Athens, GA 30605, USA.

3 Departamento de Oceanografa Fsica, Centro de Investigacin Cientfica y de Educacin Superior de Ensenada, Carretera Ensenada-Tijuana No. 3918, Fraccionamiento Playitas, Ensenada, CP 22860, Baja California, Mxico.

4 Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Am Handelshafen 12, Bremerhaven, 27570, Germany.

* Corresponding author. E-mail: [email protected]

ABSTRACT. An unusual combination of features makes the Midriff Islands region of the northern Gulf of California (NGC) a strongatmospheric methane source. Oceanographic isolation by a series of sills and islands along with upward transport of nutrient-rich waterenhanced by tidal currents, upwelling, and overflows results in high productivity. The resulting high phytoplankton biomass likely stimulatesbiogeochemical cycling that, in turn, may stimulate biological methane production in the water column and sediments. Additionally, venting ofabiogenic methane-rich hydrothermal fluids in this tectonically active area and seepage of biogenic or thermogenic methane gas from thesediments may also be important sources. We found elevated methane concentrations throughout our study area, the highest within the BallenasChannel, which was supersaturated with respect to atmospheric methane at all depths. Our vertical methane profiles show that elevateddissolved methane concentrations in the NGC are mainly associated with Gulf of California Water (GCW). Data from 22 stations suggestsouthward advection of methane via the methane-rich GCW, and lower methane concentrations south of the sills area. Our observations ofsupersaturated methane concentrations at all stations and all depths in the Ballenas Channel suggest that it is a strong source of methane to theatmosphere and to other parts of the NGC. In particular, station 7 at 50, 20, and 0 m depths had methane (CH4) concentrations of 49.1, 48.3, and43.5 nM, respectively, corresponding to saturation values of 2090%, 2050%, and 1850%, respectively. Our calculated NGC fluxes ranged from3.4 to 103.4 mol CH4 m2 d1. The average methane flux calculated for our entire study area was 21.1 mol CH4 m2 d1. These values arehigher than those measured at many other high productivity sites worldwide including upwelling sites, and suggest input of methane viahydrothermal fluids or seepage from the sediments.

Key words: dissolved methane, gas chromatography, potential methane sources and origin.

RESUMEN. Una combinacin inusual de rasgos oceanogrficos hacen de la regin de las grandes islas del norte del golfo de California (NGC)una importante fuente de metano hacia la atmsfera. El aislamiento oceanogrfico por umbrales e islas y el transporte vertical de agua rica ennutrientes, aumentado por mareas, surgencias y desbordes, resultan en alta productividad. La alta biomasa fitoplanctnica probablementeestimula el reciclado biogeoqumico, que a su vez estimula la produccin biolgica de metano en el agua y sedimento. Adicionalmente, elventeo de fluidos hidrotermales ricos en metano en esta zona tectnicamente activa y las filtraciones de gas metano, termognico o biognico,del sedimento tambin pueden ser fuentes importantes. Se encontraron concentraciones elevadas de metano a lo largo del rea de estudio,principalmente dentro del canal de Ballenas, con supersaturacin respecto al metano atmosfrico en todas las profundidades. Nuestros perfilesverticales de metano muestran que las elevadas concentraciones de metano en el NGC estn asociadas al Agua del Golfo de California (AGC).Datos de 22 estaciones sugieren una adveccin de metano al sur va AGC rica en metano, y concentraciones menores de metano al sur de losumbrales. La supersaturacin en el canal de Ballenas sugiere que ste es una fuente importante de metano hacia la atmsfera y hacia otraspartes del NGC. En particular, la estacin 7 a 50, 20 y 0 m de profundidad present concentraciones de metano (CH4) de 49.1, 48.3 y 43.5 nM,respectivamente (saturaciones de 2090%, 2050% y 1850%, respectivamente). El flujo de gas hacia la atmsfera vari de 3.4 a 103.4 mol CH4m2 d1, con un promedio total para toda el rea de 21.1 mol CH4 m2 d1. Estos valores son ms altos comparados con aquellos medidos enotros sitios de alta productividad a nivel mundial, incluyendo zonas de surgencias, y sugieren una entrada de metano va procesos hidrotermaleso filtraciones desde el sedimento.

Palabras clave: metano disuelto, cromatografa de gases, origen y fuentes potenciales de metano.

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INTRODUCTION

Methane is an important greenhouse gas that contributesabout 15% (Amouroux et al. 2002) to 21% (Houghton et al.1990) of the total radiative forcing from long-lived green-house gases. Although the atmospheric methane concentra-tion has been generally rising since the mid-20th century,there was an approximately decade-long interval duringwhich methane concentrations exhibited little change, afterwhich concentrations again began increasing (Rigby et al.2008). Berner et al. (2003) have suggested that to understandthese variations in the atmospheric methane concentrations,we must better establish the variability in the strengths ofmethane sources and sinks. Estimates of the oceanic contri-bution to the global methane budget range from 5 to 50 Tgper year. This large uncertainty reflects the need for moredetailed evaluation of marine areas with a potentially largecontribution to the methane cycle, including areas with fre-quent upwelling events (Rehder et al. 2002, Kock et al. 2008)that stimulate high productivity and facilitate transfer ofmethane from the ocean to the atmosphere. Further, thenorthern Gulf of California (NGC) is known to be tectoni-cally active and have high hydrothermal activity. When com-bined with the organic-rich sediments resulting from the highproductivity and shallow water depths, these characteristicsprovide additional mechanisms for producing methane in thesediments, and facilitating its release to the water column andpotentially the atmosphere (Campbell and Gieskes 1984,Canet et al. 2010).

It has been estimated that as much as 80% of the totalglobal methane source of 600 Tg CH4 yr1 results from bio-logical production by methanogens and the remaining 20%from geological sources (e.g., Kvenvolden and Rogers 2005).Methanogens are members of the strictly anaerobic Archaea,which compete with sulfate-reducing bacteria for reducingequivalents. Several potential sources may contribute meth-ane to the marine water column, including biological produc-tion in the upper sediments and biological production withinsuspended particulates and the intestinal tracts of organismssuch as fish and zooplankton (e.g., Traganza et al. 1979,Martens and Klump 1980, Kelley et al. 1990, de Angelisand Lee 1994, Tilbrook and Karl 1995, Kock et al. 2008).Abiogenic processes may be an important source of methanein tectonically active areas with higher than normal heat flow.In unsedimented areas, such as mid-ocean spreading centers,methane may be extracted from hot basalt by circulating sea-water (e.g., Welhan 1988), or it may be formed by Fischer-Tropsch reduction reactions associated with serpentinization(e.g., Proskurowski et al. 2008). In tectonically active areaswith sediment cover, such as Guaymas Basin, thermaldecomposition of sediment organic matter can be the domi-nant source of methane to the water column if gas transportpathways from the subsurface to the surface are available(e.g., Whelan and Lupton 1987). Physical processes such asdissolution of methane bubbles released by seepage and

INTRODUCCIN

El metano es un importante gas de efecto invernadero queaporta del 15% (Amouroux et al. 2002) al 21% (Houghtonet al. 1990) del forzamiento radiativo total debido a los gasesde invernadero de larga duracin. Aunque la concentracinde metano atmosfrico ha ido en aumento desde mediadosdel siglo XX, hubo un intervalo de aproximadamente unadcada durante el cual las concentraciones de metanoexhibieron pocos cambios, despus del cual empezaron aincrementar de nuevo (Rigby et al. 2008). Berner et al.(2003) sugirieron que para entender estas variaciones de lasconcentraciones de metano atmosfrico, es necesario conocermejor la variabilidad de las fuentes y sumideros de metano.Las estimaciones de la contribucin ocenica al presupuestoglobal del metano varan de 5 a 50 Tg por ao. Esta granincertidumbre refleja la necesidad de una evaluacin msdetallada de las zonas marinas con una aportacin potencial-mente grande al ciclo del metano, incluyendo las regionesdonde frecuentemente se presentan eventos de surgencia(Rehder et al. 2002, Kock et al. 2008) que promueven unaalta productividad y facilitan la transferencia de metano delocano a la atmsfera. Es bien conocido que la regin nortedel golfo de California (GC) es tectnicamente activa ypresenta intensa actividad hidrotermal. Estas caractersticas,junto con los sedimentos ricos en materia orgnica resultan-tes de la alta productividad y profundidades de aguassomeras, proporcionan mecanismos adicionales para producirmetano en los sedimentos y facilitar su liberacin en lacolumna de agua y potencialmente la atmsfera (Campbell yGieskes 1984, Canet et al. 2010).

Se ha estimado que hasta el 80% de la emisin mundialde metano de 600 Tg CH4 ao1 se debe a la produccinbiolgica realizada por las arqueas metangenas y el 20%restante a fuentes geolgicas (e.g., Kvenvolden y Rogers2005). Las bacterias metangenas pertenecientes al dominioArchaea son estrictamente anaerobias y compiten con lasbacterias reductoras de sulfato para los equivalentes reducto-res. Existen varias fuentes potenciales que pueden aportarmetano a la columna de agua, incluyendo la produccin bio-lgica en los sedimentos superiores y la produccin biolgicadentro del material particulado suspendido y el tracto intesti-nal de organismos como peces y zooplancton (e.g., Traganzaet al. 1979, Martens y Klump 1980, Kelley et al. 1990, deAngelis y Lee 1994, Tilbrook y Karl 1995, Kock et al. 2008).Los procesos abiognicos pueden ser una fuente importantede metano en reas tectnicamente activas con flujo de caloranormal. En reas no sedimentarias, como los centros de dis-persin ocenicos, el metano puede ser extraido del basaltocaliente por el agua de mar circulante (e.g., Welhan 1988), opuede ser formado por las reacciones reductoras de Fischer-Tropsch asociadas a la serpentinizacin (e.g., Proskurowskiet al. 2008). En reas tectnicamente activas con coberturasedimentaria como la cuenca de Guaymas, la descomposi-cin trmica de la materia orgnica del sedimento puede ser

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advection of water masses rich in methane may contribute tothe methane concentration in a particular water column(Leifer et al. 2006). Biological consumption is an importantsink of methane in the marine environment. Under oxicconditions, bacterial oxidation is the main sink (e.g., Wardand Kilpatrick 1993). Several modes of anaerobic oxidationof methane (AOM) have been documented (Joye 2012),including by consortia of anaerobic methanotrophic Archaea(ANME) and either sulfate-reducing bacteria (Orphan et al.2001) or metal oxide-reducing bacteria (Beal et al. 2009), byan oxigenic bacterium that converts nitrite to N2 and O2 anduses the O2 to oxidize methane (Ettwig et al. 2010), and by anANME that can couple AOM to disulfide disproportionation(Milucka et al. 2012).

Oceanographic areas with a high productivity or thosewith frequent upwelling events are often strong sources ofmethane to the atmosphere (Kock et al. 2008). The NGC(fig. 1) has been shown to exhibit high primary production(Zeitzschel 1969, lvarez-Borrego and Lara-Lara 1991) inresponse to enhanced upward transport of nutrient-rich water.This enhanced productivity results in a high downward fluxof particulate organic matter, decomposition of which maycontribute to methane production in the sediments. The NGCbegins just south of the large Midriff Islands (Tiburn, SanLorenzo, and San Esteban) and extends northward (fig. 1).The exchange of water between the NGC and the southerngulf is limited by the presence of sills of variable depth, ofwhich the San Lorenzo (400 m depth) and San Esteban(600 m depth) sills (Padilla et al. 2006) are the mostimportant.

Geological setting and gas seepage in the Gulf of California

The Gulf of California (GC) was formed by separation ofthe Baja California peninsula from the continent and subse-quent drift of the peninsula to the northwest (Lonsdale 1989).An important determinant of GC structural geology is atectonically-active system of long en echelon transform faultslinked by pull-apart basins, which is part of the Pacific-NorthAmerican plate boundary. These pull-apart basins wereformed at short spreading centers, many of which are stillactive, including Guaymas Basin, which is about 100 kmsoutheast of our study area. A combination of highly produc-tive surface waters and input of riverine clastic sediments hasled to a high rate of sedimentation and a 1.5- to 2-km-thickdeposit of organic-rich sediments in Guaymas Basin(Merewether et al. 1985). This rapid deposition of sedimentsinhibits seafloor eruption, such as that observed at mid-oceanridges, and intrusion of hot basaltic sills and dikes into theorganic-rich sediments is the main igneous activity (Lonsdaleand Becker 1985). The basaltic intrusions create a majorgeothermal anomaly (Lawver et al. 1975) and hydrothermalactivity (Einsele et al. 1980), including generation of

la fuente principal de metano a la columna de agua si existenrutas para transportar el gas de la subsuperficie a la superficie(e.g., Whelan y Lupton 1987). Procesos fsicos como ladisolucin de burbujas de metano liberadas por la filtracin yadveccin de masas de agua ricas en metano pueden contri-buir a la concentracin de metano en una columna de agua enparticular (Leifer et al. 2006). El consumo biolgico es unsumidero important de metano en el ambiente marino. Encondiciones xicas, la oxidacin bacteriana es el principalsumidero (e.g., Ward y Kilpatrick 1993). Se han documen-tado varios modos de oxidacin anaerobia de metano (OAM)(Joye 2012), incluyendo por consorcios de arqueas anaero-bias metanotrficas (ANME) y ya sea bacterias reductoras desulfato (Orphan et al. 2001) o bacterias reductoras de xido(Beal et al. 2009), por una bacteria oxignica que convierte elnitrito en N2 y O2 y utiliza el O2 para oxidar el metano(Ettwig et al. 2010), y por un ANME que puede acoplarOAM a la desproporcionacin de disulfuro (Milucka et al.2012).

Regiones oceanogrficas de alta productividad o desurgencias frecuentemente actan como fuentes de metano ala atmsfera (Kock et al. 2008). Se ha demostrado que laregin norte del GC (fig. 1) presenta alta productividad pri-maria (Zeitzschel 1969, lvarez-Borrego y Lara-Lara 1991)en respuesta a un mayor desplazamiento ascendente de aguarica en nutrientes. Este aumento de la productividad resultaen un alto flujo descendente de materia orgnica particulada,cuya descomposicin puede contribuir a la produccin demetano en el sedimento. La regin norte del GC comienzajusto al sur de las grandes islas (Tiburn, San Lorenzo y SanEsteban) y se extiende hacia el norte (fig. 1). El intercambiode agua entre las regiones norte y sur del GC est limitadopor la presencia de umbrales de profundidad variable, de loscuales San Lorenzo (400 m de profundidad) y San Esteban(600 m de profundidad) son los ms importantes (Padillaet al. 2006).

Situacin geolgica y filtracin de gas en el golfo de California

El GC se form por la separacin de la pennsula de BajaCalifornia del continente y su subsecuente desplazamientohacia el noroeste (Lonsdale 1989). Una importante determi-nante de la estructura geolgica del GC es un sistema tect-nicamente activo de largas fallas transformantes en echelonasociadas a cuencas de traccin (pull-apart basins), el cual sesita en el lmite de las placas del Pacfico y Norteamericana.Estas cuencas de traccin se formaron en centros de disper-sin cortos, muchos de los cuales an son activos, incluyendola cuenca de Guaymas que se ubica a unos 100 km al surestede nuestra zona de estudio. Una combinacin de aguassuperficiales muy productivas y la entrada de sedimentosclsticos fluviales ha resultado en una tasa de sedimentacinalta y un depsito de sedimentos ricos en materia orgnica de1.5 a 2 km de espesor en la cuenca de Guaymas (Merewether


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methane and other hydrocarbons by thermal decompositionof sedimentary organic matter (Whelan and Lupton 1987) inGuaymas Basin. The rate of thermogenic methane productionin Guaymas Basin was found to be so great that it completelymasked the abiogenic methane production that would beexpected in this active spreading center based on observed3He anomalies (Whelan and Lupton 1987). As a consequenceof the high methane production rate, many methane-richthermal plumes have been documented in Guaymas Basin,some extending at least 900 m above the 15002000 m deepseafloor (Merewether et al. 1985). More recently, massiveseafloor gas seepage from depths of 65 to 150 m was reportedin the pull-apart Wagner and Consag basins (Canet et al.2010), which are located about 100 km northwest of ourstudy area. Known tectonically-active zones in our study areaare the Ballenas Transform Fault (BTF) system within theBallenas Channel (BC), and Delfn Basin. The BTF is part ofthe plate boundary and is thought to be the major locus ofpresent transform movement in the Midriff Islands region(Bischoff and Henyey 1974), and the Delfn Basin containsan active spreading center (Lonsdale 1989, Persaud et al.2003).

et al. 1985). Esta rpida deposicin de sedimentos inhibe laerupcin del fondo marino, como se observa en las dorsalesocenicas, y la intrusin de umbrales y diques baslticoscalientes en los sedimentos ricos en materia orgnica es laprincipal actividad gnea (Lonsdale y Becker 1985). Lasintrusiones baslticas producen una fuerte anomala geo-trmica (Lawver et al. 1975) y actividad hidrotrmica(Einsele et al. 1980), incluyendo la generacin de metano yotros hidrocarburos por la descomposicin de materiaorgnica sedimentaria (Whelan y Lupton 1987) en la cuencade Guaymas. Se ha documentado que la tasa de produccinde metano termognico en la cuenca de Guaymas es tanrpida que enmascara la produccin de metano abiognicoque cabra esperar en este centro de dispersin activo conbase en las anomalas de 3He observadas (Whelan y Lupton1987). Como consecuencia de la alta tasa de produccin demetano, se han observado muchas plumas trmicas ricas enmetano en la cuenca de Guaymas, algunas extendindoseunos 900 m por arriba del lecho marino de 1500 a 2000 m deprofundidad (Merewether et al. 1985). Ms recientemente, seregistr una masiva filtracin de gas de profundidades de 65a 150 m en las cuencas de traccin de Wagner y Consag(Canet et al. 2010), las cuales se localizan a unos 100 km alnoroeste del rea de estudio. Las zonas tectnicamente acti-vas conocidas en el rea de estudio son el sistema de fallatransformante Ballenas (FTB) en el canal de Ballenas (CB) yla cuenca Delfn. El sistema de FTB se sita en el lmite deplacas y se considera el sitio principal del movimiento de pla-cas tectnicas en la regin de las grandes islas (Bischoff yHenyey 1974). La cuenca Delfn contiene un centro de dis-persin activo (Lonsdale 1989, Persaud et al. 2003).

Masas de agua

Las masas de agua muestreadas en este estudio (fig. 2)han sido descritas previamente (ver, por ejemplo, Bray 1988,Delgadillo-Hinojosa et al. 2001). En la parte sur de la zona deestudio, la masa de agua ms profunda fue la de Agua Inter-media del Pacfico (AIP), que se localiz entre 500 y 1000 mde profundidad. A profundidades intermedias (100 a 500 m)se encontr Agua Subsuperficial Subtropical (ASS) directa-mente sobre AIP en toda la zona de estudio. Agua del Golfode California (AGC) predomin a profundidades de 100 a200 m en la zona de estudio, aunque en una estacin se regis-tr Agua Superficial Tropical (AST). Delgadillo-Hinojosaet al. (2001) han demostrado que ASS y probablemente AIPpueden contribuir agua rica en nutrientes al agua superficialde la regin de las grandes islas del GC cuando se intensificala mezcla vertical y, as, estimular la productividad primaria.

Circulacin en la regin norte del golfo de California

Es bien conocido que existe un intercambio tipo estuarinoentre las regiones norte y sur del GC, en el cual agua rica ennutrientes, ms fra y menos salina, que origina en el ocano

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Water masses

The water masses sampled in this study (fig. 2) have beenpreviously described (see for example Bray 1988,Delgadillo-Hinojosa et al. 2001). In the south, the deepestwater mass was Pacific Intermediate Water (PIW), found at500 to 1000 m depths. Subsurface Subtropical Water (SSW)directly overlies the PIW and occupies the intermediatedepths (100 to 500 m) throughout the study area. Gulf ofCalifornia water (GCW) dominates the upper 100 to 200 m ofthe study area, although Tropical Surface Water (TSW) wasobserved at one station. Delgadillo-Hinojosa et al. (2001)have shown that SSW and probably PIW can contributenutrient-rich water to the surface water in the Midriff Islandsregion of the GC via enhanced vertical mixing, and thusstimulate high primary productivity.

Circulation in the northern Gulf of California

It has long been recognized that the NGC has anestuarine-like exchange with the southern gulf, in whichcolder, less saline, nutrient-rich water originating in thePacific Ocean (mainly SSW) inflows from the south at depth,and warmer, more saline, nutrient-depleted water (mainlyGCW) outflows near the surface. This is believed to be adirect consequence of net heat gain and evaporation of theNGC, which requires export of relatively warm and salinewater and import of fresher and colder water to maintain heatand salt balances (Bray 1988, Lavn and Organista 1988).

Recently, direct current measurements at the San Estebanand San Lorenzo sills (see fig. 3) have estimated the net near-bottom inflow to be 0.18 Sv (1 Sv = 1 106 m3 s1), equallydivided between the two sills (Lpez et al. 2006). The flowthrough the San Lorenzo sill (near station 30) descends to thedeepest part of the BC in the form of an overflow, mixingvigorously as it descends from sill-depth (~400 m) to thedeepest part of the basin (~1600 m). The flow through theSan Esteban sill continues through the Tiburn Basin, even-tually flowing over the Delfn sill (see fig. 3) and descendinginto Delfn Basin, the second deepest basin (~800 m) in theNGC (Lpez et al. 2008). Delfn Basin is connected to BCvia the BC sill at 600 m depth (see fig. 3). Therefore, the twodeepest basins in the NGC receive 0.18 Sv of water, which iscontinually discharged to their deepest parts. Both basins areclosed below 400 m depth and connected by the BC sill.Therefore, water from the two-basin system cannot outflowbelow 200 m because the entering overflows extend 100 to200 m above the bottom (400 m depth at the San Lorenzo andDelfn sills). A simple mass balance calculation taking intoaccount the volume of both basins, their surface area, and the0.18 Sv near-bottom inflow, results in a mean verticalvelocity of approximately 5 m per day extending upwards towithin at least 300 m of the surface in the BC and DelfnBasin (Lpez et al. 2008). These vertical velocities are typi-cal of strong upwelling areas, but in the NGC these are mean

Pacfico (principalmente ASS), fluye de sur a norte cerca delfondo, mientras que agua pobre en nutrientes, ms caliente yms salina (principalmente AGC) fluye de norte a sur cercade la superficie. Esto parece ser una consecuencia directa dela ganancia neta de calor y evaporacin en la regin norte delGC, lo cual requiere la exportacin de agua relativamentecaliente y salina, y la importacin de agua menos salina yms fra para mantener los balances de sal y calor (Bray1988, Lavn y Organista 1988).

Mediciones directas de corrientes en los umbrales deSan Esteban y San Lorenzo (ver fig. 3) han mostrado que elflujo hacia el norte cerca del fondo es de 0.18 Sv (1 Sv =1 106 m3 s1), dividido equitativamente entre los dos umbra-les (Lpez et al. 2006). El flujo a travs del umbral de SanLorenzo (cerca de la estacin 30) desciende hasta la partems profunda del CB, causando un mezcla vigorosa con-forme desciende de la profundidad del umbral (~400 m) a laparte ms profunda de la cuenca (~1600 m). El flujo a travsdel umbral de San Esteban contina por la cuenca Tiburn yeventualmente fluye sobre el umbral Delfn (ver fig. 3) ydesciende en la cuenca Delfn, la segunda cuenca ms

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Figure 2. T-S diagram of all the CTD casts performed during theUmbrales III campaign in August 2003. Contours are for -1000,where is potential density calculated at atmospheric pressure.TSW and PDW stand for Tropical Surface Water and Pacific DeepWater, respectively. The definitions for GCW, Gulf of CaliforniaWater; SSW, Subsurface Subtropical Water; and PIW, PacificIntermediate Water follow those of Torres-Orozco (1993) andCastro et al. (2006).Figura 2. Diagrama T-S de todos los lances de CTD realizadosdurante la campaa de muestreo (Umbrales III) en agosto de 2003.Los contornos son para -1000, donde es la densidad potenciala la presin atmsferica. Los acrnimos TSW y PDW significanAgua Tropical Superficial y Agua Profunda del Pacfico,respectivamente. Las definiciones de GCW, Agua del Golfo deCalifornia; SSW, Agua Subtropical Subsuperficial; y PIW, AguaIntermedia del Pacfico siguen las de Torres-Orozco (1993) yCastro et al. (2006).


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velocities, not just peak velocities during strong, wind-forcedupwelling events. This rapid upward transport of nutrient-rich water is responsible for the high biological productivityof the NGC, particularly BC that receives nutrient-rich, near-bottom SSW at both ends (Lpez et al. 2006). A near-surfaceoutflow of water must counterbalance the net deep inflowand associated upwelling in the BC and Delfn Basin. Thesenear-surface outflows have been measured at the BC and SanLorenzo and San Esteban sills (fig. 3). At the BC sill thenear-surface flow is located between 50 and 300 m depth andis directed out of the channel towards the northwest (Lpezet al. 2006). A schematic representation of the near-bottomand near-surface circulation at the sills is shown in figure 3.

In this paper we report and evaluate dissolved methaneconcentrations in the Midriff Islands region of the NGC. Weused the data to calculate sea-air methane fluxes to betterunderstand the importance of marine areas characterized byhigh productivity and/or upwelling in the global methanecycle. We hypothesized that the NGC might be a strongmethane source due to several characteristics: (1) highprimary productivity and associated rapid biogeochemicalcycling, (2) strong upward transport of methane-rich waterfrom depth, and (3) the possible presence of thermogenicmethane associated with hydrothermal and tectonic activityin relatively shallow (300 to 400 m depth) areas. We also

profunda (~800 m) en la regin norte del GC (Lpez et al.2008). La cuenca Delfn se conecta con el CB mediante elumbral del canal a una profundidad de 600 m (ver fig. 3). Porlo tanto, las dos cuencas ms profundas en la regin norte delGC reciben 0.18 Sv de agua, que es constantemente transpor-tada a sus partes ms profundas. Ambas cuencas estn cerra-das por debajo de los 400 m de profundidad y conectadas porla cuenca del CB. Por ende, el agua en este sistema de doscuencas no puede fluir hacia afuera por debajo de los 200 mporque los flujos entrantes se extienden 100 a 200 m porarriba del fondo (400 m de profundidad en los umbrales deSan Lorenzo y Delfn). Un clculo de balances de materiasimple tomando en cuenta el volumen de ambas cuencas, surea superficial y el flujo cerca del fondo de 0.18 Sv, arrojauna velocidad vertical media de aproximadamente 5 m porda extendindose hacia arriba hasta por lo menos 300 m dela superficie del CB y la cuenca Delfn (Lpez et al. 2008).Estas velocidades verticales son tpicas de reas de surgenciafuerte, pero en la regin norte del GC son velocidades mediasy no slo velocidades mximas durante fuertes eventos desurgencia inducida por el viento. Este rpido transporteascendente de agua rica en nutrientes es responsable de la altaproductividad biolgica en el norte del GC, especialmente enel CB que recibe ASS rica en nutrientes cerca del fondo enambos extremos (Lpez et al. 2006). Una salida de aguasuperficial debe contrarrestar la entrada neta de agua pro-funda y la surgencia asociada en el CB y la cuenca Delfn.Este flujo hacia afuera cerca de la superficie ha sido medidoen el CB y en los umbrales de San Lorenzo y San Esteban(fig. 3). En el umbral de CB, el flujo cerca de la superficie selocaliza entre 50 y 300 m de profundidad y es dirigida fueradel canal hacia el noroeste (Lpez et al. 2006). En la figura 3se presenta una representacin esquemtica de la circulacincerca del fondo y cerca de la superficie a travs de losumbrales.

En el presente trabajo se analizan las concentraciones demetano disuelto registradas en la regin de las grandes islasdel GC. Se usaron los datos para calcular los flujos de metanoentre el mar y la atmsfera para entender mejor la importan-cia de zonas marinas caracterizadas por una alta productivi-dad y/o surgencias en el ciclo global del metano. Se plantela hiptesis de que la regin norte del GC podra ser unafuente de metano debido a varias caractersticas: (1) la altaproductividad primaria y el rpido reciclado biogeoqumicoasociado, (2) el fuerte transporte ascendente de agua rica enmetano del fondo y (3) la posible presencia de metano termo-gnico asociado con la actividad hidrotrmica y tectnica enzonas relativamente someras (300 a 400 m de profundidad).Tambin se analiza el uso potencial del metano como un tra-zador para diferenciar masas de agua y elucidar los flujos enel rea de los umbrales.

MATERIALES Y MTODOS

Se obtuvieron muestras de agua en agosto de 2003,durante la tercera fase de una campaa realizada en la zona

Figure 3. Map of the northern Gulf of California showing majorbasins and sills. Schematic representation of near-bottom (bluearrows) and near-surface (red arrows) flows. The black symbolsrepresent the San Lorenzo (), San Esteban (), Delfn (), andBallenas Channel () sills. Isobaths are shown as solid lines.Figura 3. Mapa de la regin norte del golfo de Californiamostrando las cuencas y los umbrales principales. Representacinesquemtica de los flujos cerca del fondo (flechas azules) y cercade la superficie (flechas rojas). Los smbolos negros representanlos umbrales de San Lorenzo (), San Esteban (), Delfn () ydel canal de Ballenas (). Las isbatas se indican como lneascontinuas.

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investigate the potential use of methane as a tracer inhelping differentiate water masses and elucidate flows in thesills area.

MATERIALS AND METHODS

Water samples were collected in August 2003 during thethird phase of the Umbrales campaign. The samples werecollected at standard depths (0, 10, 20, 50, 100, 200, 300,400, 500 m and so on) at 24 stations using 5-L Go-Flo andNiskin bottles (fig. 1). Samples were then carefully trans-ferred with minimal exposure to air to amber 250-mL bottles,which were filled to the brim, poisoned with HgCl2, andtightly capped. Samples were kept cool and in the dark, andtransported to the lab for analysis.

Methane quantification

The dissolved methane in each sample was determinedusing the phase equilibrium method between an aqueoussample and an inert gas phase (McAuliffe 1971, Capasso andInguaggiato 1998) at standard conditions (25 C, 1 atm).High purity helium was used as the inert gas. Quantificationwas carried out using a HP-6890 Series II gas chromatographequipped with a flame ionization detector and a six-wayvalve with a 1-mL sample loop for sample introduction.We used a 15-m-long HP-PLOT/Molsieve column with a0.53-mm internal diameter and 50-m film thickness. Thewater vapor was removed by using a short piece of tubing(about 5 cm long) filled with active Drierite.

A seven-point calibration curve was derived from work-ing standards, which were prepared by adding varyingamounts of a 1000 ppb methane standard (Scott Marrin, Inc.)with a gas-tight syringe to known volumes of high purity Hein Kevlar gas sampling bags. All working standards wereinjected five times, and average peak areas (in pA*s) werecalculated and used for the calibration. A typical calibrationcurve is shown in figure 4.

After the calibration was performed, a 50-mL samplefrom each bottle was transferred to a gas-tight syringe. Eachsyringe was then placed in a water bath at 25 C for 15 min;20 mL of sample were removed from each syringe andreplaced with 20 mL of high purity He. The syringes werethen vigorously shaken for 15 min with a wrist-action shaker.Since methane is very insoluble in water (McAuliffe 1971,Wiesenburg and Guinasso 1979), it is rapidly transferredfrom the liquid phase to the gas phase, and shaking for morethan 5 min results in transfer of > 90% of the methane to theheadspace. The headspace methane concentration was thenused to calculate the sample in situ methane concentration(Johnson et al. 1990). Based on the calibration curve, weestimated a limit of detection of < 0.1 nM. The uncertainty,measured from duplicate samples, was estimated to be ~21%.

The saturation rate of methane (SR%) was then calculatedby dividing the measured concentration (Cmeasured) at the

de umbrales (Umbrales III). Las muestras se recolectaron aprofundidades estndar (0, 10, 20, 50, 100, 200, 300, 400,500 m y as sucesivamente) en 24 estaciones con botellas Go-Flo y Niskin de 5 L. Las muestras fueron transferidas cuida-dosamente, con mnima exposicin al aire, a botellas de colormbar de 250 mL, las cuales fueron llenadas hasta el tope,envenenadas con HgCl2 y hermticamente cerradas. Lasmuestras se mantuvieron frescas y en la oscuridad, y se trans-portaron al laboratorio para su anlisis.

Cuantificacin del metano

Se determin el metano disuelto en cada muestramediante el mtodo de equilibrio de fases entre una muestraacuosa y una fase de gas inerte (McAuliffe 1971, Capasso yInguaggiato 1998) en condiciones estndar (25 C, 1 atm). Seus helio de alta pureza como el gas inerte. La cuantificacinse realiz usando un cromatgrafo de gases HP-6890 Serie IIequipado con un detector de ionizacin de llama y unavlvula de seis vas provista de un bucle de muestra de 1 mLpara introducir la muestra. Se us una columna HP-PLOT/Molsieve de 15 m de largo con un dimetro interno de0.53 mm y espesor de pelcula de 50 m. El vapor de agua seretir con un trozo de tubo corto (~5 cm de largo) lleno deDrierite activo.

Se prepar una curva de calibracin de siete puntos deri-vada de estndares de trabajo mediante la adicin, con unajeringa hermtica a los gases, de cantidades variables de unestndar de metano de 1000 ppb (Scott Marrin, Inc.) avolmenes conocidos de He en bolsas para muestreo Kevlar.Todos los estndares fueron inyectados cinco veces, y lospromedios de las reas de los picos (en pA*s) fueron calcula-dos y usados para la calibracin. En la figura 4 se muestrauna curva de calibracin tpica.

y x = 0.012 + 0.560R = 0.995

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surface by the concentration that should exist if in equilib-rium with the atmosphere (Ceq), and multiplying by 100:

(1)

This equilibrium concentration (Ceq) was estimated usingthe polynomial derived by Wiesenburg and Guinasso (1979).Saturation values above 100 indicate a flux from the sea tothe atmosphere.

Methane flux calculations

The flux of methane (F in mol m2 d1) from the oceanto the atmosphere was calculated by:

(2)

where k is the gas transfer coefficient (m d1), which was cal-culated using a wind speed (u) of 5.6 m s1 for all methaneflux calculations and using the Schmidt number (Sc) follow-ing the parameterization of Sweeney et al. (2007). The Sc is afunction of the seawater temperature, gas diffusion coeffi-cient, and water kinematic viscosity, and was calculatedusing the third-order polynomial expression derived byWanninkhof (1992). The solubility of methane at equilibriumwas calculated using the temperature and salinity dependencefit derived by Wiesenburg and Guinasso (1979). A windspeed of 5.6 m s1 obtained from QuikSCAT was used for allmethane flux calculations as mentioned above. A positivevalue of F indicates net methane flux from the ocean to theatmosphere. We did not measure the atmospheric methaneconcentration during the cruise and assume that it was1775 ppbv (Sansone et al. 2004, Steele et al. 2002) for allcalculations.

RESULTS

Temperature-salinity profiles obtained during theUmbrales III campaign in 2003 (fig. 2) clearly show thewater masses that were sampled in the NGC. The profilescorrespond to the stations shown in figure 1.

The BC sill area (stations 7 and 8) appears to be a hotspotfor methane production in the Midriff Islands region. Thehighest methane concentrations were found in near-surfacesamples in the BC-Delfn Basin area (figs. 58). In particular,station 7 at 50, 20, and 0 m depths had methane concentra-tions of 49.1, 48.3, and 43.5 nM, respectively, which corre-spond to saturation values of 2090%, 2050%, and 1850%,respectively (fig. 5). Moreover, the whole BC water columnis supersaturated with methane. The highest surface methaneconcentration was observed at station 7, slightly north of theBC sill, which separates Delfn Basin from the BC (fig. 6).The horizontal methane fields suggest northwest to southeasttransport along the BC and transport towards the northeastaway from the BC sill at both the surface (fig. 6) and 50 m

SR% = Cmeasured Ceq 100

F k Cmeasured Ceq =

Despus de realizar la calibracin, se transfiri unamuestra de 50 mL a una jeringa hermtica a los gases. Cadajeringa fue colocada en un bao de agua a 25 C durante15 min. Se retiraron 20 mL de muestra de cada jeringa y sereemplazaron con 20 mL de He de alta pureza. Las jeringasfueron agitadas vigurosamente durante 15 min con un agita-dor de accin de mueca. Ya que el metano es muy insolubleen agua (McAuliffe 1971, Wiesenburg y Guinasso 1979) setransfiere rpidamente de la fase lquida a la gaseosa y su agi-tacin por ms de 5 min produce una transferencia de > 90%del metano al espacio libre superior. La concentracin demetano en el espacio libre se us para calcular la concentra-cin de metano in situ de la muestra (Johnson et al. 1990).Con base en la curva de calibracin, se estim un lmite dedeteccin de < 0.1 nM. La incertidubre, medida de muestrasduplicadas, se estim en ~21%.

La tasa de saturacin de metano (TS%) se calcul divi-diendo la concentracin medida (Cmedida) en la superficie entrela concentracin que debera existir si estuviera en equilibriocon la atmsfera (Ceq) y multiplicando por 100:

(1)

Esta concentracin de equilibrio (Ceq) se estim medianteel polinomio derivado por Wiesenburg y Guinasso (1979).Los valores de saturacin por encima de 100 indican un flujodel mar a la atmsfera.

Estimacin del flujo de metano

El flujo de metano (F en mol m2 d1) del ocano a laatmsfera se calcul como sigue:

(2)

donde k es el coeficiente de transferencia del gas (m d1), elcual se calcul usando la velocidad del viento de 5.6 m s1para todos los cculos de flujo de metano y usando el nmerode Schmidt (Sc) siguiendo la parameterizacin de Sweeney etal. (2007). El Sc es una funcin de la temperatura del agua demar, el coeficiente de difusin del gas y la viscosidad cine-mtica del agua, y se calcul con la expresin del polinomiode tercer orden derivada por Wanninkhof (1992). La solubili-dad del metano en equilibrio se estim mediante el ajuste dedependencia entre la temperatura y la salinidad derivado porWiesenburg y Guinasso (1979). Se us una velocidad delviento de 5.6 m s1 obtenida de QuikSCAT para todos losclculos de flujo de metano, tal como se menciona arriba. Unvalor positivo de F indica un flujo neto de metano del ocanoa la atmsfera. No se midi la concentracin atmosfrica delmetano durante el crucero y se supuso que sta fue de1775 ppbv (Sansone et al. 2004, Steele et al. 2002) para todaslas estimaciones.

TS% = Cmedida Ceq 100

F k Cmedida Ceq =


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depth (fig. 7). The unusual hydrography of the BC, in whichnear-surface outflows of upwelled water exit at both ends ofthe channel to counterbalance the inflows, probably explainsthese features of the horizontal methane fields. In addition, adeeper (300 m) methane maximum was detected at station 38(fig. 5), which is located to the south of the sills and near thefringe of the Guaymas Basin (figs. 6, 9).

RESULTADOS

Los perfiles de temperatura y salinidad obtenidos durantela campaa Umbrales III en 2003 (fig. 2) muestran clara-mente las masas de agua muestreadas en la regin norte delGC. Los perfiles corresponden a las estaciones que se mues-tran en la figura 1.

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Figure 5. Vertical distributions of dissolved methane in the water column for stations marked. Subsurface maximums are visible at allstations. The maximum for station 38 at 300 m has been attributed to the presence of an oxygen minimum zone.Figura 5. Distribuciones verticales del metano disuelto en la columna de agua de las estaciones indicadas. Los mximos subsuperficiales sonvisibles en todas las estaciones. El mximo para la estacin 38 a 300 m ha sido atribuida a la presencia de una zona de mnimo de oxgeno.


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A north to south methane section along stations 3, 14, 13,11, 22, 24, 28, 34, and 38 is shown in figure 9. GCW appearsto be transporting methane southwards along the middle partof the NGC in the near-surface waters. The relatively highmethane concentrations at stations 14, 13, and, to a lesserdegree, 11, appear to be the result of advection from the BCsill area (station 7). The high concentrations coming from theBC sill area are transported in a fan-like pattern to the north,east, and south on the eastern side of ngel de la GuardaIsland. Since the transport is very near the surface ( 100 m),association with GCW is indicated. The 35 salinity isolineseparates the GCW from the SSW (fig. 10). That isoline isfound at depths > 200 m north of the sills. Advection of meth-ane southwards (fig. 9) is mainly due to GCW. This watermass is found from the surface to around 200 m depth. The300 m depth methane maximum at station 38 is just visible atthe edge of figure 9, and a decreasing methane gradienttowards the north and towards the surface away from thismaximum is suggested.

We used the mean wind speed of 5.6 m s1 obtained fromQuickscat, and the approach described above to calculate thesea-air methane flux for all of our stations. All stations except22 and 24 (which showed a slight undersaturation) weresupersaturated with respect to the atmospheric equilibriumconcentration. Our observed sea-air methane fluxes ranged

La zona del umbral de CB (estaciones 7 y 8) parece ser unrea crtica para la produccin de metano en la regin de lasgrandes islas. Las mayores concentraciones de metano seencontraron en las muestras recolectadas cerca de la superfi-cie en la zona del CB y la cuenca Delfn (figs. 58). Enparticular, la estacin 7 present concentraciones de 49.1,48.3 y 43.5 nM a 50, 20 y 0 m de profundidad, respectiva-mente, que corresponden a valores de saturacin de 2090%,2050% y 1850%, respectivamente (fig. 5). Adems, toda lacolumna de agua del CB se encuentra sobresaturada demetano. La mayor concentracin de metano superficial seregistr en la estacin 7, un poco al norte del umbral de CB,que separa la cuenca Delfn del CB (fig. 6). Los campos hori-zontales de metano sugieren un transporte del noroeste alsureste a lo largo del CB y un transporte hacia el noreste delCB en tanto la superficie (fig. 6) como a 50 m de profundidad(fig. 7). La hidrografa inusual del CB, donde los flujossalientes de agua aflorada cerca de la superficie en ambosextremos del canal contrarrestan los flujos entrantes, proba-blemente explica estas caractersticas de los campos horizon-tales de metano. Tambin se detect un mximo de metanoms profundo (300 m) en la estacin 38 (fig. 5), localizada alsur de los umbrales y cerca del borde de la cuenca deGuaymas (figs. 6, 9).

En la figura 9 se muestra una seccin de metano de nortea sur a lo largo de las estaciones 3, 14, 13, 11, 22, 24, 28, 34 y38. El AGC parece estar transportando metano en agua cercade la superficie hacia el sur. Las concentraciones relativa-mente altas en las estaciones 14, 13 y, en menor grado,11,parecen ser resultado de la adveccin de la zona del umbralde CB (estacin 7). Las concentraciones altas procedentes de

Figure 6. Surface methane (CH4) concentration in the northernGulf of California. Only the deep blue color represents zones withundersaturation of methane. Largest supersaturation was observedat the Ballenas Channel sites. Figura 6. Concentracin superficial de metano (CH4) en la reginnorte del golfo de California. Slo el color azul oscuro representalas zonas con subsaturacin de metano. La mayor sobresaturacinse observ en los sitios del canal de Ballenas.

Figure 7. Methane (CH4) concentration at 50 m depth.Figura 7. Concentracin de metano (CH4) a 50 m de profundidad.


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from 3.4 to 103.4 mol m2 d1 (table 1). The highest fluxeswere observed for stations 7, 8, 9, and 14, with values of103.4, 79.5, 39.3, and 37.5 mol CH4 m2 d1, respectively.Stations 7, 8, and 9 are all located in the BC, where there isstrong vertical mixing and high surface water methane con-centrations (figs. 8, 11).

Subsurface maxima were located at or near the pycno-cline usually around 50 m depth. In several instances, tworelative methane maxima were observed in the vertical pro-files (see for example fig. 5, stations 7 and 38). For stations 3,10, 21 (not shown), and 7, 28 and 38, the two methane max-ima suggest more than one source of this gas. For stations 10and 28, the methane maxima were observed at the surfaceand 50 m depth. Methane maxima were located at 0 and200 m for station 21 and at 20 and 300 m for station 38.

DISCUSSION AND CONCLUSIONS

The measured average methane concentration for BC sur-face waters was 23.4 nM (saturation about 1000%), whichresults in an estimated methane flux of 74.1 mol m2 d1.This is a very large value when compared to other valuesreported elsewhere (table 1). Assuming that BC has an areaclose to 270,000 ha, during summer the above flux wouldextrapolate to about 3.2 t CH4 d1 from BC alone. An estimatefor the whole sampling area using the average surface con-centration for all surface samples yields about 4.9 t d1, whichemphasizes that BC is a hotspot of methane release in theNGC. We understand that spatial and temporal constraintslimit the representativeness of our methane concentrationdata and the resultant estimated fluxes. This work was carriedout on a ship of opportunity, and it was only possible toobtain one set of samples over a limited area.

la zona del CB son transportadas en forma de abanicohacia el norte, este y sur por el lado oriental de la isla ngelde la Guarda. El transporte es muy cerca de la superficie( 100 m), lo que indica una asociacin con el AGC. La iso-lnea de salinidad de 35 separa el AGC del ASS (fig. 10). Talisolnea se encuentra a profundidades > 200 m al norte de losumbrales. La adveccin de metano hacia el sur (fig. 9) sedebe principalmente al AGC. Esta masa de agua se encuentradesde la superficie hasta unos 200 m de profundidad. La con-centracin mxima de metano a 300 m en la estacin 38 esapenas visible en el margen de la figura 9, y se supone ungradiente decreciente hacia el norte y hacia la superficie deeste mximo.

Se usaron la velocidad media del viento obtenida deQuickscat y el enfoque descrito anteriormente para calcularel flujo de metano del ocano a la atmsfera para todaslas estaciones. Todas las estaciones excepto 22 y 24 (quemostraron una ligera subsaturacin) se encontraron sobresa-turadas con respecto a la concentracin en equilibrio con laatmsfera. Nuestros flujos de metano ocano-atmsferaobservados variaron de 3.4 a 103.4 mol m2 d1 (tabla 1).Los mayores flujos de 103.4, 79.5, 39.3 y 37.5 mol CH4 m2d1 se observaron en las estaciones 7, 8, 9 y 14, respectiva-mente. Las estaciones 7, 8 y 9 se ubican en el CB, donde hay

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Figure 8. Methane (CH4) section along the Ballenas Channel.Figura 8. Seccin de metano (CH4) a lo largo del canal deBallenas.

Figure 9. Methane (CH4) section from north to south alongstations 3, 14, 13, 11, 22, 24, 28, 34, and 38 (see fig. 1) in thenorthern Gulf of California. The deep blue color represents zoneswith methane undersaturation. Largest saturation was observed at50 m depth. GCW, Gulf of California Water; TSW, TropicalSurface Water; SSW, Subsurface Subtropical Water; PIW, PacificIntermediate Water.Figura 9. Seccin de metano (CH4) de norte a sur a lo largo de lasestaciones 3, 14, 13, 11, 22, 24, 28, 34 y 38 (ver fig. 1) en laregin norte del golfo de California. El color azul oscurorepresenta las zonas con subsaturacin de metano. La mayorsaturacin se observ a 50 m de profundidad. GCW, Agua delGolfo de California; TSW, Agua Superficial Tropical; SSW, AguaSubtropical Subsuperficial; PIW, Agua Intermedia del Pacfico.

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Processes that could be responsible for the methaneconcentrations that we observed in the NGC include biologi-cal production in the water column and sediments, thermaldecomposition of sedimentary organic matter, and abiogenicproduction. With only methane concentration measurementsavailable we cannot unambiguously distinguish among thesepossibilities. High rates of methane production coupled tointense biogeochemical cycling associated with high primaryproductivity stimulated by extensive inflow of nutrient-richwater over the sills at both ends of the BC (Lpez et al. 2006)likely contribute to the generally elevated BC methane con-centrations. The position of the maximum and the shape ofthe methane distribution gradients observed in figure 8, andmore clearly in figure 9, suggest an in situ biological methaneproduction in the water column, most likely in zooplanktonguts (e.g., de Angelis and Lee 1994).

Although in situ methane production is probably animportant contributor to the relatively modest levels of super-saturation observed throughout much of the NGC, a seafloorsource, such as gas seepage from the sediments or methane-rich hydrothermal fluids, is likely required to explain the verylarge saturation values detected in the BC. The station 9profile (fig. 5), for example, shows a slight methane concen-tration increase with depth near the bottom, suggesting apossible sediment source. Evidence of hydrothermal activityin our study area includes an anomalously high sediment heatflow measurement in the BC within about 10 km of ourstation 8 (Lawver et al. 1975) and several volcanic knolls thathave pierced the seafloor very near our stations 5 and 7

fuerte mezcla vertical y altas concentraciones de metano en elagua superficial (figs. 8, 11).

Se localizaron mximos subsuperficiales en o cerca de lapicnoclina generalmente alrededor de los 50 m de profundi-dad. En varios casos, se observaron dos mximos relativos demetano en los perfiles verticales (ver, por ejemplo, fig. 5,estaciones 7 y 38). En las estaciones 3, 10, 21 (no se mues-tran) y 7, 28 y 38, los dos mximos de metano sugieren msde una fuente de este gas. En las estaciones 10 y 28, losmximos de metano se observaron en la superficie y a 50 mde profundidad. Los mximos de metano se localizaron a 0 y200 m en la estacin 21, y a 20 y 300 m en la estacin 38.

DISCUSIN Y CONCLUSIONES

La concentracin promedio de metano medido para lasaguas superficiales del CB fue de 23.4 nM (saturacin dealrededor de 1000%), lo que resulta en un flujo de metanoestimado de 74.1 mol m2 d1. Este valor es muy alto encomparacin con los valores registrados por otros autores(tabla 1). Suponiendo que el CB tiene un rea de cerca de270,000 ha, durante el verano este flujo se extrapolara a 3.2 tCH4 d1 de tan solo el CB. Una estimacin ms amplia detoda la zona usando la concentracin promedio de todas lasmuestras superficiales arroja un valor de alrededor de 4.9 td1, lo cual demuestra que el CB es un rea crtica de libera-cin de metano en la parte norte del GC. Entendemos querestricciones espaciales y temporales limitan la representati-vidad de nuestros datos sobre la concentracin de metano ylos flujos resultantes estimados. Este trabajo se llev a caboen un crucero de oportunidad y slo fue posible obtener unconjunto de muestras en un rea limitada.

Figure 11. Methane (CH4) concentration of surface samplesversus water temperature. Ballenas Channel (BC) stations(numbers) showed the largest methane concentrations.Figura 11. Concentracin de metano (CH4) en muestrassuperficiales versus temperatura del agua. Las mayoresconcentraciones de metano se registraron en las estaciones(nmeros) del canal de Ballenas (BC).

0.05.0

10.015.020.025.030.035.040.045.050.0

26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 30.5 31.0

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1314

15

Figure 10. Salinity section along Tiburn Basin and San Estebansill. Station numbers are marked above and north is at the left ofthe figure. The limit between Gulf of California Water andSubsurface Subtropical Water is marked by the 35 isohaline.Figura 10. Seccin de salinidad en la cuenca Tiburn y el umbralde San Esteban. Los nmeros de las estaciones se indican en laparte superior y el norte se ubica a la izquierda de la figura. Laisohalina de 35 indica el lmite entre el Agua del Golfo deCalifornia y el Agua Subtropical Subsuperficial.

Stations3 14 11 22 28 3813 24 34

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(Persaud et al. 2003). Faulting associated with the BTFsystem and intrusion of hot basalt into the upper sedimentlayers (e.g., volcanic knolls) would provide conduits for gastransport from the subsurface to the surface. Due to recentvolcanic activity and/or strong bottom currents, much of theBTF has < 40 m of sediment fill (Bischoff and Henyey 1974,Lonsdale 1989) compared to an estimated 1.5 to 2 km of sed-iment fill in Guaymas Basin. In areas of the BTF system withlittle or no sediment cover, we would expect generation ofthermogenic methane to be less overwhelmingly importantthan it is in Guaymas Basin and abiogenic methane produc-tion to play a larger role. Assuming that the inferred methaneseep is located in the BTF system and from an abiogenicsource, we speculate that Fischer-Tropsch reduction reactionsassociated with serpentinization (e.g., Proskurowski et al.2008) would be a more likely mechanism than extraction ofmethane from hot basalt by circulating sea water (Welhan1988) because the serpentinization mechanism generallyproduces much higher methane concentrations (e.g., Charlouet al. 2010). If the inferred methane seep is located in DelfnBasin, then thermogenic methane production is more likely.Sedimentation rate in Delfn Basin is very high due to thehigh primary productivity of the area (Zeitzschel 1969,lvarez-Borrego and Lara-Lara 1991) and its proximity tothe Colorado River. The Delfn Basin sedimentation rate dur-ing the last 100 years ranged from 2.4 to 3.8 cm yr1 (Baba etal. 1991), which is about one order of magnitude higher thanthe Guaymas Basin sedimentation rate (Merewether et al.1985). The high sedimentation rate would tend to protect theorganic matter from biological decomposition, and where anintrusion of hot basalt occurs there would be the potential for

Los procesos que podran ser responsables de las concen-traciones de metano observadas en la regin norte del GCincluyen la produccin biolgica en la columna de agua ysedimentos, la descomposicin trmica de la materiaorgnica sedimentaria y la produccin abiognica. No esposible distinguir claramente entre estas posibilidades conbase en slo las mediciones de la concentracin de metanodisponibles. Altas tasas de produccin de metano junto conun reciclado biogeoqumico intensivo asociado con la altaproductividad primaria estimulada por la entrada de agua ricaen nutrientes a travs de los umbrales en ambos extremos delCB (Lpez et al. 2006) probablemente contribuyen a lasgeneralmente elevadas concentraciones de metano en el CB.La posicin del mximo y la forma de los gradientes de dis-tribucin de metano observadas en la figura 8, y con mayorclaridad en la figura 9, sugieren una produccin biolgica demetano in situ en la columna de agua, probablemente en losintestinos de zooplancton (e.g., de Angelis y Lee 1994).

Si bien la produccin de metano in situ probablementesea un contribuidor importante a los niveles relativamentemodestos de supersaturacin observados en casi toda laregin norte del GC, para explicar los elevados valores desaturacin detectados en el CB se necesitara una fuente delfondo marino, como las filtraciones de gas de los sedimentoso fluidos hidrotermales ricos en metano. El perfil de la esta-cin 9 (fig. 5), por ejemplo, muestra un incremento ligero demetano hacia el fondo, lo que sugiere una posible fuente sedi-mentaria. La evidencia de actividad hidrotermal en la zona deestudio incluye una medida anmalamente alta de flujo decalor de sedimentos en el CB a unos 10 km de nuestra esta-cin 8 (Lawver et al. 1975) y varios montculos volcnicos

Table 1. Reported sea-air methane fluxes from different marine environments. Comparison to values obtained in this work is difficult due todifferences in oeanographic conditions. Values are shown as intervals or as the mean standard error.Tabla 1. Flujos de metano del ocano a la atmsfera registrados para diferentes ambientes marinos. Una comparacin con los valores obtenidosen el presente trabajo es difcil debido a las diferentes condiciones oceanogrficas. Los valores se muestran como intervalos o como la media el error estndar.

Geographic location Reference Flux (mol CH4 m2 d1)V-4 Vortex Tilbrook and Karl (1995) 0.93.5Mexican Pacific, all stations Sansone et al. (2004) 0.55.9Atlantic, overall transect Kelley and Jeffrey (2002) 0.38 0.32Sargasso Sea Holms et al. (2000) 1.64.4Sea of Japan Gamo et al. (2012) 2.64 1.92East China Sea Zhang et al. (2008) 36.6 95.7Yellow Sea Zhang et al. (2004) 1.33 0.76Pearl River Estuary Zhou et al. (2009) 63.5 32.2Northern South China Sea Zhou et al. (2009) 15.6 8.0Archaeon Lagoon, high-tide, France Deborde et al. (2010) 4.840.8Whole study area This work 21.1 5.0Whole study area (range) This work 3.4103.4 21.7


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thermogenic methane production. An unknown variable isthe sediment organic matter content, which, if too low due tohigh influx of Colorado River sediments, could limit ther-mogenic methane production. If no hydrothermal activity ispresent, then anoxic diagenesis of sedimentary organic mattercan also form methane gas in the sediments, which may bereleased as bubbles to the water column. Release of methanebubbles from the sediments could be triggered (Canet et al.2010) by the extensive faulting known to be present in theBC sill area (Persaud et al. 2003). Measurements of gas com-position (i.e., ethane and propane) and methane stable isotoperatios could help to clarify the relative importance of thepotential biogenic, thermogenic, and abiogenic sources andcould also help constrain the importance of methane oxida-tion, but we were unable to obtain these measurements withour samples.

Methane advection from the south appears to be minor. Aslight gradient from station 38 at 300 m depth (fig. 9), whichis somewhat visible in figures 5 and 6, was the only indica-tion of methane advection from the south that we observed.Campbell and Gieskes (1984) estimated a water columnmethane anomaly of about 4 M from dilution of GuaymasBasin vent waters, and Merewether et al. (1985) observedthat the plumes could extend to at least 900 m above the bot-tom, so it is conceivable that advection of methane-rich waterfrom Guaymas Basin could account for the station 38 maxi-mum of ~20 nM. This small methane source, however, doesnot appear to reach across the sills into the northern stations.A longer lateral transport distance in oxygenated waters willnecessarily translate into more complete in situ methane oxi-dation and relatively smaller fluxes to the atmosphere. Theobserved methane gradients are able to suggest water massmovement, as is evident from figures 58, and can be helpfulin cases where several different water masses mix, as occursin the NGC.

We observed unusually large methane concentrations andfluxes in the sills region of the NGC during a single samplingcampaign. Additional data, such as the higher hydrocarbon(e.g., ethane and propane) concentrations and the isotopicsignature of methane, with multiple sampling campaignsunder a range of oceanographic conditions, more completesampling over a wider area, and denser sampling of hotspotsare needed to clarify the origin and cycling of methane andthe importance of the area as an atmospheric methane source.Gas seepage from the sediments is likely to be highly local-ized and would be difficult to detect by fixed station watercolumn sampling. Underway echo sounding (e.g., Canet etal. 2010) and/or deploying a towed or autonomous underwa-ter vehicle with CTD and METS methane sensors (e.g.,Merewether et al. 1985, Newman et al. 2008) would be amore efficient means of detecting methane and hydrothermalplumes over a wide area, such as the NGC, and would behelpful in pinpointing the locations of individual seeps,including the inferred seep near the BC sill.

que han perforado el fondo marino muy cerca de nuestrasestaciones 5 y 7 (Persaud et al. 2003). Fallas asociadas con elsistema de FTB y la intrusin de basalto caliente en las capassuperiores de sedimento (e.g., montculos volcnicos) pro-porcionaran conductos para transportar gas de la subsuperfi-cie a la superficie. Debido a la reciente actividad volcnicay/o las fuertes corrientes del fondo, gran parte del FTB tiene< 40 m de relleno sedimentario (Bischoff y Henyey 1974,Lonsdale 1989) en comparacin con 1.5 a 2 km de rellenosedimentario estimado para la cuenca de Guaymas. En reasdel sistema de FTB con poca o nula cobertura sedimentariacabra esperar que la generacin de metano termognico seramucho menos importante que en la cuenca de Guaymas y quela produccin de metano abiognico tendra un papel msimportante. Suponiendo que la filtracin de metano inferidase encuentra en el sistema de FTB y es de una fuenteabiognica, especulamos que las reacciones reductoras deFischer-Tropsch asociadas con la serpentinizacin (e.g.,Proskurowski et al. 2008) seran un mecanismo ms probableque la extraccin de metano de basalto caliente por el aguade mar circulante (Welhan 1988), ya que el mecanismo deserpentinizacin generalmente produce concentraciones demetano mucho mayores (e.g., Charlou et al. 2010). Si lafiltracin de metano se encuentra en la cuenca Delfn, enton-ces la produccin de metano termognico sera ms probable.La tasa de sedimentacin en la cuenca Delfn es muy altadebido a la elevada productividad primaria en la regin(Zeitzschel 1969, lvarez-Borrego y Lara-Lara 1991) y a suproximidad al ro Colorado. Durante los pasados 100 aos latasa de sedimentacin en la cuenca Delfn oscil entre 2.4 y3.8 cm ao1 (Baba et al. 1991), lo cual es alrededor de unorden de magnitud mayor que la tasa de sedimentacin en lacuenca de Guaymas (Merewether et al. 1985). Una tasa desedimentacin alta tendera a proteger la materia orgnica dedescomposicin biolgica, y donde se produce una intrusinde basalto caliente habra el potencial para la produccin demetano termognico. Una variable desconocida es el conte-nido de materia orgnica en los sedimentos, ya que si es muybajo debido a la entrada de sedimentos del ro Coloradopodra limitar la produccin de metano termognico. Si noexiste actividad hidrotermal, entonces la diagnesis anxicade la materia orgnica sedimentaria tambin puede formargas metano en los sedimentos, el cual puede ser liberado en lacolumna de agua en la forma de burbujas. La liberacin deburbujas de metano de los sedimentos puede ser activada(Canet et al. 2010) por las numerosas fallas que se encuen-tran en el umbral del CB (Persaud et al. 2003). Medicionesde la composicin de gases (i.e., etano y propano) y las pro-porciones de istopos estables de metano podran ayudar aesclarecer la importancia relativa de las fuentes biognicas,termognicas y abiognicas potenciales, as como la impor-tancia de la oxidacin de metano, pero no fue posible obtenerestas mediciones con nuestras muestras.

Parece haber poca adveccin de metano del sur. La nicaindicacin de adveccin del sur que se observ fue un ligero


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ACKNOWLEDGMENTS

Although this work was reviewed by the United StatesEnvironmental Protection Agency and approved for publica-tion, it might not necessarily reflect Agency policy. Mentionof trade names or commercial products does not constituteendorsement or recommendation for use. We would like toexpress our thanks to the National Council for Science andTechnology (CONACYT, Mexico) for partially financing theproject under contract #G33464-T. We would also like tothank the crew of the oceanographic vessel Francisco deUlloa for helping during the field work.

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Se observaron flujos y concentraciones de metanoinusualmente altas en la zona de umbrales de la regin nortedel GC durante una sola campaa de muestreo. Para esclare-cer el origen y ciclo del metano, y la importancia de la regincomo una fuente de metano atmsferico, se requieren datosadicionales tales como las concentraciones de otros hidrocar-buros gaseosos (e.g., etano y propano) y la huella isotpicadel metano, as como mltiples campaas de muestreo en unavariedad de condiciones oceanogrficas, y muestreos mscompletos en una zona ms amplia y en zonas crticas. La fil-tracin de gas de los sedimentos suele ser muy localizada ysera difcil de detectar mediante el muestreo de estacionesfijas en la columna de agua. Sondeos ultrasnicos (e.g., Canetet al. 2010) y/o el uso de vehculos submarinos autnomos oremolcados con sensores de metano CTD y METS (e.g.,Merewether et al. 1985, Newman et al. 2008) seran mediosms eficaces para detectar el metano y las plumas hidroter-males en una regin amplia como el norte del GC, y serantiles para ubicar las filtraciones individuales, incluyendo lafiltracin inferida en el umbral de CB.

AGRADECIMIENTOS

Aunque este trabajo fue revisado por la Agencia deProteccin Ambiental de los Estados Unidos y aprobado parasu publicacin, no necesariamente refleja la poltica de laagencia. La mencin de marcas o productos comerciales noconstituye aprobacin o recomendacin para su uso. Este tra-bajo fue parcialmente financiado por el Consejo Nacional deCiencia y Tecnologa (Mxico) bajo el contrato #G33464-T.Agradecemos a la tripulacin del barco oceanogrficoFrancisco de Ulloa su apoyo durante el trabajo de campo.

Traducido al espaol por Christine Harris.


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Received September 2012,received in revised form January 2013,

accepted February 2013.

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