Biogeosciences, 15, 5715–5731, 2018 https://doi.org/10.5194/bg-15-5715-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. On the formation of hydrothermal vents and cold seeps in the Guaymas Basin, Gulf of California Sonja Geilert1, Christian Hensen1, Mark Schmidt1, Volker Liebetrau1, Florian Scholz1, Mechthild Doll2, Longhui Deng3, Annika Fiskal3, Mark A. Lever3, Chih-Chieh Su4, Stefan Schloemer5, Sudipta Sarkar6, Volker Thiel7, and Christian Berndt1 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1–3, 24148 Kiel, Germany 2Faculty of Geosciences, University of Bremen, Klagenfurter-Straße 4, 28359 Bremen, Germany 3Department of Environmental Systems Science, ETH Zurich, Universitätstrasse 16, 8092 Zurich, Switzerland 4Institute of Oceanography, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan 5Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany 6Department of Earth and Climate Science, Indian Institute of Science Education and Research Pune, Dr. Homi Bhabha Road, Maharashtra-411008, India 7Geobiology, Geoscience Centre, University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany Correspondence: Sonja Geilert ([email protected]) Received: 8 January 2018 – Discussion started: 6 February 2018 Revised: 4 September 2018 – Accepted: 5 September 2018 – Published: 27 September 2018 Abstract. Magmatic sill intrusions into organic-rich sedi- lated that deep fluid and thermogenic gas flow ceased 28 to ments cause the release of thermogenic CH4 and CO2. Pore 7 kyr ago. These findings imply a short lifetime of hydrother- fluids from the Guaymas Basin (Gulf of California), a sed- mal systems, limiting the time of unhindered carbon release imentary basin with recent magmatic activity, were investi- as suggested in previous modeling studies. Consequently, ac- gated to constrain the link between sill intrusions and fluid tivation and deactivation mechanisms of these systems need seepage as well as the timing of sill-induced hydrothermal to be better constrained for the use in climate modeling ap- activity. Sampling sites were close to a hydrothermal vent proaches. field at the northern rift axis and at cold seeps located up to 30 km away from the rift. Pore fluids close to the active hy- drothermal vent field showed a slight imprint by hydrother- mal fluids and indicated a shallow circulation system trans- 1 Introduction porting seawater to the hydrothermal catchment area. Geo- chemical data of pore fluids at cold seeps showed a mainly Abrupt climate change events in Earth’s history have been ambient diagenetic fluid composition without any imprint re- partly related to the injection of large amounts of greenhouse lated to high temperature processes at greater depth. Seep gases into the atmosphere (e.g., Svensen et al., 2004; Gut- communities at the seafloor were mainly sustained by mi- jahr et al., 2017). Among the most prominent of these events crobial methane, which rose along pathways formed earlier was the Paleocene–Eocene Thermal Maximum (PETM) dur- ◦ by hydrothermal activity, driving the anaerobic oxidation of ing which the Earth’s atmosphere warmed by about 8 C in methane (AOM) and the formation of authigenic carbonates. less than 10 000 years (Zachos et al., 2003). The PETM was Overall, our data from the cold seep sites suggest that possibly triggered by the emission of about 2000 Gt of car- at present, sill-induced hydrothermalism is not active away bon (Dickens, 2003; Zachos et al., 2003). The processes dis- from the ridge axis, and the vigorous venting of hydrothermal cussed regarding the release of these large amounts of car- fluids is restricted to the ridge axis. Using the sediment thick- bon in a relatively short time are gas hydrate dissociation, ness above extinct conduits and carbonate dating, we calcu- volcanic eruptions as well as igneous intrusions into organic- rich sediments, triggering the release of carbon during con- Published by Copernicus Publications on behalf of the European Geosciences Union. 5716 S. Geilert et al.: On the formation of hydrothermal vents and cold seeps tact metamorphism (Svensen et al., 2004; Aarnes et al., 2010; cline just as quickly (< 10 kyr) (Bani-Hassan, 2012; Iyer et Gutjahr et al., 2017). al., 2017). The Guaymas Basin in the Gulf of California is consid- During the expedition SO241 by RV SONNE in June/ July ered one of the few key sites to study carbon release in a rift 2015 a new hydrothermal vent field was discovered at the basin exposed to high sedimentation rates. A newly discov- flank of the northern trough (Fig. 1; Berndt et al., 2016). The ered vent field in the Guaymas Basin, which releases large discovered mound rises up to 100 m above the seafloor and amounts of CH4 and CO2 up to several hundreds of meters predominant black-smoker-type vents suggest similar end- into the water column (Berndt et al., 2016), stimulated the member temperatures and geochemical composition found discussion on the climate potential of magmatic intrusions at the southern trough (Von Damm et al., 1985; Von Damm, into organic-rich sediments (e.g., Svensen et al., 2004). 1990; Berndt et al., 2016). The hydrothermal vent system The Gulf of California is located between the Mexican emits methane-rich fluids with a helium isotope signature mainland and the Baja California peninsula, north of the East indicative of fluids in contact with mid-ocean ridge basalt Pacific Rise (EPR; Fig. 1). The spreading regime at EPR con- (Berndt et al., 2016). On this cruise, we sampled this re- tinues into the Gulf of California and changes from a ma- cently discovered hydrothermal vent field and some of the ture, open-ocean type to an early opening continental rifting off-axis seeps above sill intrusions described by Lizarralde et environment with spreading rates of about 6 cm yr−1 (Cur- al. (2010). The aim of this study was to investigate the fluid ray and Moore, 1982). Its spreading axis consists of two and gas compositions of the off-axis seeps in order to identify graben systems (northern and southern troughs) offset by a the influence of sill intrusions on fluid circulation, gas com- transform fault (Fig. 1). The Guaymas Basin, which is about position, and the timing of hydrothermal activity. The overall 240 km long, is around 60 km wide, reaches water depths motivation was thus to explore the regional and temporal ex- of up to 2000 m, and is known as a region of vigorous hy- tent of hydrothermal activity in the area and to provide better drothermal activity (e.g., Curray and Moore, 1982; Gieskes constraints on carbon release from sedimented ridge systems. et al., 1982; Von Damm et al., 1985). Hydrothermal activ- ity in the Guaymas Basin was first reported in the southern trough (e.g., Lupton, 1979; Gieskes et al., 1982; Campbell 2 Materials and methods and Gieskes, 1984; Von Damm et al., 1985). Here, fluids em- anate partly from black-smoker-type vents at temperatures 2.1 Sampling devices and strategy of up to 315 ◦C (Von Damm et al., 1985). The rifting envi- During the RV SONNE expedition SO241, seven sites across ronment in the Guaymas Basin shows a high sediment accu- the central graben of the Guaymas basin were investigated mulation rate of up to 0.8–2.5 m kyr−1, resulting in organic- (Fig. 1). Site-specific sampling and data recording were per- rich sedimentary deposits of several hundreds of meters in formed using (1) a video-guided multi-corer (MUC), (2) a thickness (e.g., Calvert, 1966; DeMaster, 1981; Berndt et al., gravity corer (GC), (3) temperature loggers attached to a GC 2016). The high sedimentation rate is caused by high biolog- or sediment probe, (4) a video-guided VCTD/Rosette wa- ical productivity in the water column and an influx of ter- ter sampler, and (5) a video-guided hydraulic grab (VgHG). rigenous matter from the Mexican mainland (Calvert, 1966). Sites were selected according to published data on the lo- Sills and dikes intruding into the sediment cover have a sub- cations of seeps (Lizarralde et al., 2010) and seismic data stantial impact on the distribution of heat flow, other environ- acquired during the cruise (see below). mental conditions, and thus early diagenetic processes within the basin (Biddle et al., 2012; Einsele et al., 1980; Kast- 2.1.1 Seismic data recording ner, 1982; Kastner and Siever, 1983; Simoneit et al., 1992; Lizarralde et al., 2010; Teske et al., 2014). Seismic data were collected using a Geometrics GeoEel Magmatic intrusions and cold seeps at the seafloor were Streamer of 150 and 183.5 m length and 96 and 112 channels, observed up to 50 km away from the rift axis, and a re- respectively. Two generator-injector guns in harmonic mode cently active magmatic process triggering the alteration of (105/105 cubic inch) served as the seismic source. Process- organic-rich sediments and releasing thermogenic CH4 and ing included navigation processing (1.5625 m crooked line CO2 was proposed by Lizarralde et al. (2010). These au- binning), 20, 45, 250, and 400 Hz frequency filtering, and thors attributed elevated CH4 concentrations and tempera- post-stack Stolt migration with water velocity yielding an ap- ture anomalies in the water column to active thermogenic proximately 2 m horizontal and 5 m vertical resolution close CH4 production driven by contact metamorphism. Accord- to the seafloor. ing to Lizarralde et al. (2010) ongoing off-axis hydrothermal − activity may cause a maximum carbon flux of 240 kt C yr 1 2.1.2 Sediment and pore fluid sampling through the seafloor into the ocean and potentially into the at- mosphere. However, modeling studies investigating the life- At seepage and vent sites, the video-guided MUC was used time of such sill-induced hydrothermalism show that initial to discover recent fluid release, which was indicated by typ- CH4 and CO2 release is intense and vigorous but can de- ical chemosynthetic biological communities at the seafloor Biogeosciences, 15, 5715–5731, 2018 www.biogeosciences.net/15/5715/2018/ S.