A source of isotopically light organic carbon in a low-pH anoxic marine zone

Abstract

Geochemical and stable isotope measurements in the anoxic marine zone (AMZ) off northern Chile during periods of contrasting oceanographic conditions indicate that microbial processes mediating sulfur and nitrogen cycling exert a significant control on the carbonate chemistry (pH, AT, DIC and pCO2) of this region. Here we show that in 2015, a large isotopic fractionation between DIC and POC, a DIC and N deficit in AMZ waters indicate the predominance of in situ dark carbon fixation by sulfur-driven autotrophic denitrification in addition to anammox. In 2018, however, the fractionation between DIC and POC was significantly lower, while the total alkalinity increased in the low-pH AMZ core, suggesting a predominance of heterotrophic processes. An isotope mass-balance model demonstrates that variations in the rates of sulfur- and nitrogen-mediated carbon fixation in AMZ waters contribute ~7–35% of the POC exported to deeper waters. Thus, dark carbon fixation should be included in assessments of future changes in carbon cycling and carbonate chemistry due to AMZ expansion.

Isotopic Characterization of Water Masses in the Southeast Pacific Region: Paleoceanographic Implications

“…In the coastal section the PDW/ESSW/AAIW mixing zone (Figure 4) exhibits relatively lower δ 13 C DIC values than in the oceanic section. This feature is not traceable with temperature, salinity, oxygen and phosphate, suggesting that depleted δ 13 C DIC from the OMZ/ESSW can reach deep waters and influence the distribution of this tracer in the coastal section (Silva et al., 2009; Vargas et al., 2021).…”

“…Between 200 and 600 m, the ESSW dominates the water column below the SAAW and STW in the tropical/subtropical SEP. This water mass is characterized by high salinity and nutrient contents, very low oxygen, and low δ 13 C DIC values (Garcia et al., 2014; Quay et al., 2003; Vargas et al., 2021; Wooster & Gilmartin, 1961). The ESSW derives from the mixing of aged water masses in the (western) Equatorial Pacific and is transported eastward by the Equatorial Undercurrent (EUC).…”

Bacterial and eukaryotic intact polar lipids point to in situ production as a key source of labile organic matter in hadal surface sediment of the Atacama Trench

“…A physical-chemical characterization of the water column during the ChiMeBo-SO211, LowpHOX-2, and HADES-SO261 cruises has been reported in Matys et al (2017), Cantarero et al (2020) and Vargas et al (2021), andFernández-Urruzola et al (2021), respectively. Briefly, the potential-temperature-salinity-dissolved oxygen (θ -s-O 2 ) diagrams revealed an oxygenated and well-mixed water mass occupying the deeper parts of the Atacama Trench (Fig.…”

“…Marine trenches receive organic carbon from a variety of sources and transport mechanisms. These include canyons and river systems that channel OM from land to coastal regions, aeolian transport, surface water productivity, and in situ production, to name a few (Wenzhöfer et al, 2016;Tarn et al, 2016;Luo et al, 2017;Xu et al, 2018;Guan et al, 2019;Xu et al, 2021). Carbon flux events can increase the delivery of particulate carbon from surface waters to the seafloor (Poff et al, 2021), whereas river discharge and ae- olian transport can result in enhanced terrestrial carbon (Xu et al, 2021).…”

“…These include canyons and river systems that channel OM from land to coastal regions, aeolian transport, surface water productivity, and in situ production, to name a few (Wenzhöfer et al, 2016;Tarn et al, 2016;Luo et al, 2017;Xu et al, 2018;Guan et al, 2019;Xu et al, 2021). Carbon flux events can increase the delivery of particulate carbon from surface waters to the seafloor (Poff et al, 2021), whereas river discharge and ae- olian transport can result in enhanced terrestrial carbon (Xu et al, 2021). Mass wasting events are also known to create dynamic depositional conditions and strong spatial heterogeneity in OM distribution in marine trenches (Schauberger et al, 2021;Xu et al, 2021).…”