Hydrothermal Energy and Organic Carbon oceanic ridge crests, volcanic arcs and back-arc systems are expected to significantly influence biomass production rates. A particular challenge is to develop observing strategies that will account for the full range of environmental variables while attempting to derive global or regional estimates.

Organic salts, such as Fe, Ca, and Mg oxalates and acetates, may be widespread radiolysis and oxidation products of organic matter in Martian surface sediments. Such organic salts are challenging to identify by evolved gas analysis but the ubiquitous CO 2 and CO in pyrolysis data from the Sample Analysis at Mars (SAM) instrument suite on the Curiosity rover indirectly points to their presence. Here, we examined laboratory results from SAM-like analyses of organic salts as pure phases, as trace phases mixed with silica, and in mixtures with Ca and Mg perchlorates. Pure oxalates evolved CO 2 and CO, while pure acetates evolved CO 2 and a diverse range of organic products dominated by acetone and acetic acid. Dispersal within silica caused minor peak shifting, decreased the amounts of CO 2 evolved by the acetate standards, and altered the relative abundances of the organic products of acetate pyrolysis. The perchlorate salts scrubbed Fe oxalate CO releases and shifted the CO 2 peaks to lower temperatures, whereas with Ca and Mg oxalate, a weaker CO release was observed but the initial CO 2 evolutions were largely unchanged. The perchlorates induced a stronger CO 2 release from acetates at the expense of other products. Oxalates evolved ∼47% more CO 2 and acetates yielded ∼69% more CO 2 when the perchlorates were abundant. The most compelling fits between our organic salt data and SAM CO 2 and CO data included Martian samples acquired from modern eolian deposits and sedimentary rocks with evidence for low-temperature alteration. Plain Language SummaryIn our efforts to characterize indigenous Martian organic matter, we must contend with a near-surface record that has been substantially altered by radiation and oxidation. Under such conditions, much of the surficial organic record on Mars may have decomposed into organic salts, which are challenging for flight instruments to conclusively identify. If organic salts are widespread on the Martian surface, their composition and distribution could offer insight into the less-altered organic record at depth and they may play an important role in near-surface carbon cycling and habitability. The organic detection techniques employed by the Mars Science Laboratory Curiosity rover include thermal extraction in combination with mass spectrometry. In this work, we used laboratory thermal extraction techniques analogous to those of the rover to examine organic salts as pure standards, as minor phases in a silica matrix, and in mixtures with O 2 -evolving perchlorate salts. When we compared our results with flight data, we found that many of the CO 2 profiles produced by our organic salt samples were similar to the CO 2 evolutions observed by the rover. The best fits with our laboratory data included Martian materials acquired from modern eolian deposits and sedimentary rocks that had evidence for lowtemperature alteration.LEWIS ET AL.

In order to investigate whether geochemical, physiographic and lithological differences in two end-member sedimentary settings could evoke varied microbe-sediment interactions, two 25 cm long sediment cores from contrasting regions in the Central Indian Basin have been examined. Site TVBC 26 in the northern siliceous realm (10°S, 75AE5°E) is organic-C rich with 0AE3 ± 0AE09% total organic carbon. Site TVBC 08 in the southern pelagic red clay realm (16°S, 75AE5°E), located on the flank of a seamount in a mid-plate volcanic area with hydrothermal alterations of recent origin, is organic-C poor (0AE1 ± 0AE07%). Significantly higher bacterial viability under anaerobic conditions, generally lower microbial carbon uptake and higher numbers of aerobic sulphur oxidizers at the mottled zones, characterize core TVBC 26. In the carbon-poor environment of core TVBC 08, a doubling of the 14 C uptake, a 250 times increase in the number of autotrophic nitrifiers, a four-fold lowering in the number of aerobic sulphur oxidizers and a higher order of denitrifiers exists when compared with core TVBC 26; this suggests the prevalence of a potentially autotrophic microbial community in core TVBC 08 in response to hydrothermal activity. Microbial activity at the northern TVBC 26 is predominantly heterotrophic with enhanced chemosynthetic activity restricted to tan-green mottled zones. The southern TVBC 08 is autotrophic with increased heterotrophic activity in the deepest layers. Notably, the bacterial activity is generally dependent on the surface productivity in TVBC 26, the carbon-rich core, and mostly independent in TVBC 08, the carbon-poor, hydrothermally influenced core. The northern sediment is more organic sink-controlled and the southern sediment is more hydrothermal source-controlled. Hydrothermal activity and associated rock alteration processes may be more relevant than organic matter delivery in these deep-sea sediments. Thus, this study highlights the relative importance of hydrothermal activity versus organic delivery in evoking different microbial responses in the Central Indian Basin sediments.