On sampling the upper crustal reservoir of the NE Pacific Ocean

Abstract

The oceanic upper crustal reservoir is a 600-m thick layer of porous and permeable basaltic rock that forms the uppermost igneous basement underlying the global ocean. Pore spaces within this fluid aquifer contain a significant fraction of the global seawater, and active circulation through this reservoir has profound influence on the chemical composition of the ocean, strongly impacting the biological environment near the sea surface. Because of the relative inaccessibility of the deep seafloor, where hydrothermal fluid discharges and seawater re-charges the oceanic crustal aquifer, our understanding of the dynamic physical, chemical and biological processes is strongly dependent on our ability to obtain uncontaminated samples from this challenging environment. Recent technological advances have addressed some, but certainly not all of these sampling problems, providing new data and samples that test our current hypotheses about the crustal fluid reservoir. Current scientific interest in the sub-seafloor biosphere has focused on the uppermost igneous oceanic crust as likely to be one of the most habitable environments, because of its porosity and locus of hydrothermal circulation of chemical nutrients. Recent observations indicate that sub-seafloor crustal environments harbor novel CO 2 -utilizing bacteria (primary producers) that could be a significant source of carbon-fixation in the ocean, thus broadening possible habitable zones both on Earth and elsewhere where microbial life could exist independent of nutrient input from photosynthesis.

Introduction of TAIGA Concept

“…Such a hypothesis is supported by an observation that significant amount of water may exist within the oceanic crust (e.g., Johnson et al 2006). The compilation of the measured porosity of the oceanic crust indicates that the values are up to 34 % in the young oceanic crust and higher than 10 % even in altered crust older than 10 Ma (Johnson and Pruis 2003).…”

Activity and phylogenetic diversity of sulfate-reducing microorganisms in low-temperature subsurface fluids within the upper oceanic crust

“…Sulfate reduction rates (SRR) in unamended incubations at in situ temperatures were higher in lower temperature fluids from borehole 1025C (0.05 nmol ml −1 d −1 at 39°C) compared to in higher temperature fluids from borehole U1301A (0.01 nmol ml −1 d −1 at 63°C). The younger upper oceanic crust of JFR (borehole 1025C) experiences a more vigorous recharge of deep-sea bottom seawater through unsedimented areas than the older crust (borehole U1301A), which is covered by a thicker sediment layer (Johnson et al, 2006 ). Active recharge of the crustal aquifer with fresh bottom seawater may replenish the microbial communities near borehole 1025C with electron donors more so than those in older crust near borehole U1301A.…”

“…The upper oceanic crust is arguably the most favorable of deep-subsurface environments due to the vigorous production hydrothermal fluids and sporadic recharge of bottom seawater through unsedimented areas (Johnson et al, 2006 ). In comparison, our SRR are an order of magnitude higher than the potential SRR of ~0.002 nmol ml −1 d −1 measured for sediments at the sediment-basement interface (~262 mbsf) nearby U1301A (Engelen et al, 2008 ).…”

“…In fluids from the basaltic ocean crust, metabolisms of free-living populations related to oxygen, iron, sulfur, hydrogen, and methane cycling have been inferred from fluid alteration chemistry (Wheat et al, 2010 ; McCarthy et al, 2011 ; Orcutt et al, 2013 ; Lin et al, 2014 ) and sequence diversity in environmental DNA (Cowen et al, 2003 ; Huber et al, 2006 ; Jungbluth et al, 2013 , 2014 ). However, owing to the wide range of conditions in the ridge flanks and the severely restricted access to crustal aquifers, rates of catabolic activity have only been predicted (Johnson et al, 2006 ; Orcutt et al, 2011 , 2013 ).…”

Microbial Ecology of the Dark Ocean above, at, and below the Seafloor

“…Researchers have employed colonization chambers or unique fluid collection devices to obtain samples. One approach to collecting high-temperature venting fluids is via insertion of a sampling tube below the seafloor, either by penetration of the seafloor with a heavy probe with embedded sampling lines (230,232,252) or by inserting the open end of a sampling device directly into a vent or shimmering water (73,441) or into a crevice in the seafloor (277). This is done in an effort to avoid entrainment of seawater in the fluid samples.…”