Abstract
Bering Sea sediments represent exceptional archives, offering the potential to study past climates and biogeochemistry at a high resolution. However, abundant hydrocarbons of microbial origin, especially along the eastern Bering Sea continental margin, can hinder the applicability of palaeoceanographic proxies based on calcareous foraminifera, due to the formation of authigenic carbonates. Nonetheless, authigenic carbonates may also bear unique opportunities to reconstruct changes in the sedimentary redox environment.Here we use a suite of visual and geochemical evidence from single-specimens of the shallow infaunal benthic foraminiferal species Elphidium batialis Saidova (1961), recovered from International Ocean Discovery Program (IODP) Site U1343 in the eastern Bering Sea, to investigate the influence of authigenic carbonates on the foraminiferal trace metal composition. Our results demonstrate that foraminiferal calcite tests act as a nucleation template for secondary carbonate precipitation, altering their geochemistry where organoclastic sulphate reduction and anaerobic oxidation of methane cause the formation of low- and high-Mg calcite, respectively. The authigenic carbonates can occur as encrusting on the outside and/or inside of foraminiferal tests, in the form of recrystallization of the test wall, or as banding along natural laminations within the foraminiferal test walls. In addition to Mg, authigenic carbonates are enriched in U/Ca, Mn/Ca, Fe/Ca, and Sr/Ca, depending on the redox environment that they were formed in. Our results demonstrate that site-specific U/Ca thresholds are a promising tool to distinguish between diagenetically altered and pristine foraminiferal samples, important for palaeoceanographic reconstructions utilising the primary foraminiferal geochemistry. Consistent with previous studies, U/Mn ratios of foraminifera at IODP Site U1343 increase according to their degree of diagenetic alteration, suggesting a potential response of authigenic U/Mn to the microbial activity in turn linked to the sedimentary redox environment.
Highlights
As part of the International Ocean Discovery Program (IODP) Expedition 323 several locations were cored along the eastern Bering Sea continental margin, with high siliciclastic and organoclastic sedimentation rates indicating the opportunity to study Quaternary climates at a sub-orbital resolution
This study focuses on two narrow depth intervals of Holocene (number of samples (n) = 4) to Pleistocene (n = 12) age between 0.33–8.99 m core composite depth below seafloor (CCSF-A) and 172.02–305.63 m CCSF-A, respectively (Fig. 2) at IODP Site U1343
There are interspecies differences in diagenetic alteration, with E. batialis usually displaying stronger discolouration compared to other abundant infaunal benthic foraminiferal species at Site U1343, such as Uvigerina spp. and Islandiella norcrossi
Summary
As part of the International Ocean Discovery Program (IODP) Expedition 323 several locations were cored along the eastern Bering Sea continental margin, with high siliciclastic and organoclastic sedimentation rates indicating the opportunity to study Quaternary climates at a sub-orbital resolution. The elemental and isotopic composition of planktonic and benthic foraminifera at IODP Site U1343 could provide an unrivalled record of climate variability in a region from which there is currently a dearth of information (Kender et al, 2018). The report of cooccurrence of discoloured foraminiferal tests with authigenic carbonate crystals within the sediments of IODP Site U1343 (Expedition 323 Scientists, 2010), indicates the necessity to examine the potential influence of sedimentary diagenesis on foraminiferal geochemistry before this archive is fully utilised.
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