Post-depositional overprinting of chromium in foraminifera

Abstract

Present-day ocean deoxygenation has major implications for marine ecosystems and biogeochemical cycling in the oceans. Chromium isotopes are used as a proxy to infer changes in past oceanic redox state. Chromium isotopes in carbonates, including the prime proxy carrier foraminifera, were initially thought to record the seawater composition during crystallisation. However, the uptake of Cr into foraminiferal tests and carbonates is still poorly understood and recent studies question this assumption. We assess whether Cr in foraminiferal calcite is taken up during biomineralisation, has a postdepositional origin or is a combination of the two. Laser Ablation-MC-ICP-MS analyses and NanoSIMS imaging of individual tests were used to characterise the distribution of Cr in both planktic and benthic foraminifera. Foraminifera in sediment core-top samples have up to two orders of magnitude more Cr than sediment trap, plankton net, and culture samples. In cultured specimens, Cr is incorporated in foraminiferal tests at low concentrations (0.04-0.13 ppm) with a distribution coefficient of ~250 ± 43 (2SE) which is an upper estimate due to substantial loss of dissolved Cr during the experiment. Part of the Cr signal in sedimentary foraminifera may be primary, but this primary signal is likely often overprinted by the uptake of Cr from bottom and pore waters. In sediment samples, there is no significant isotopic offset between individual species and bulk foraminiferal calcite from the same size fraction. The >500 µm fraction has a heavier isotopic composition than the smaller 250-500 µm fraction with an offset of-0.3 to-0.5‰ due to an increase in surface area to volume. We propose that Cr in foraminifera is predominantly post-depositional and records bottom/pore water signals. This is contrary to current interpretations of the foraminiferal Cr isotope proxy as a surface seawater redox proxy.

Investigating Ocean Deoxygenation During the PETM Through the Cr Isotopic Signature of Foraminifera

“…Less oxygenated waters are expected to have a heavier δ 53 Cr due to partial isotopic fractionation under more reducing conditions (Ellis et al, ). Therefore, one may expect intermediate and deep water δ 53 Cr to reflect changes in redox state which should be reflected by foraminiferal calcite (Goring‐Harford et al, ; Remmelzwaal et al, ). The δ 53 Cr values for the PETM at ODP Site 1210 fall within the oxic and hypoxic range (Goring‐Harford et al, ).…”

“…As the δ 53 Cr records are incomplete for ODP Sites 401 and 1263, these correlations need to be approached with caution and may underestimate the Cr excursion. Additionally, when considering vital effects, the shifts in the foraminiferal δ 53 Cr likely slightly overestimate environmental changes (Remmelzwaal et al, ), since the maximum test size of planktic foraminifera may have increased by up to 150 μm during the PETM depending on the foraminiferal species although overall decreases in test size have also been reported (Alegret et al, ; Kaiho et al, ; Petrizzo, ). Based on Remmelzwaal et al (), the change in δ 53 Cr composition associated with the increase in foraminiferal test size and assemblage could account for a maximum of +0.1‰ to +0.3‰ of the Cr isotope signal during the PETM depending on the test shape if a test increase is assumed.…”

“…Samples were cleaned by gently cracking open the foraminiferal tests, with subsequent removal of clay particles and organic matter by ultrasonication in Milli‐Q (MQ) grade water and by leaching in alkali buffered H 2 O 2 , respectively (Barker et al, ). Analysis of the chemical composition of marine Fe‐Mn‐rich crusts shows that these have relatively low Cr concentrations (16–47‐ppm Cr; Hein & Koschinsky, ), and therefore, the Fe‐Mn coatings were not removed to preserve as much of the calcite as possible for analysis (Remmelzwaal et al, ).…”

“…However, recent work has shown that while some Cr is incorporated during calcification most Cr in sedimentary foraminifera has a postdepositional origin and therefore will record bottom and pore water Cr compositions (Remmelzwaal et al, ). Remmelzwaal et al () also demonstrated that foraminiferal δ 53 Cr values are not species dependent but rather dependent on foraminiferal test size meaning that the surface area/volume ratio controls Cr exchange between the test and bottom water and/or sediment pore waters. Questions around the empirical relationship between δ 53 Cr and [O 2 ] as well as potential dissolution effects remain unanswered.…”

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean

“…Therefore, the isotopic signature of Cr exported with organic carbon would reflect isotopically light Cr (III), a record which may be preserved in marine sediments. Considerable work has been conducted to assess if δ 53 Cr in carbonates, which incorporate Cr (VI) (Tang et al, ), reflects ambient seawater δ 53 Cr (Bonnand et al, ; Bruggmann et al, ; Farkaš et al, ; Frei et al, ; Holmden et al, ; Pereira et al, ; Remmelzwaal et al, ; Rodler et al, ; Wang et al, ). Similar work is needed to determine how δ 53 Cr records in oxic marine sediments relate to δ 53 Cr in the overlying water column and how faithfully these records are preserved (Figure d).…”