Publication Date:
2021-02-08
Description:
This study investigates chromium isotope variations (δ53Cr) and REE patterns in present-day biogenic carbonates and ocean waters from Lady Elliot Island (LEI) located in the southern Great Barrier Reef (GBR), Australia, which is one of the world's largest carbonate-producing shelf ecosystems. Our results from thoroughly cleaned biogenic carbonates collected at LEI, with no detectable evidence for lithogenic Cr and/or Mn–Fe oxide coating contamination, revealed a systematic and statistically significant correlation (r2=0.83, p〈0.05) between δ53Cr and cerium anomaly (Ce/Ce*) data in molluscan shells (i.e., gastropods). This in turn implies a redox-controlled incorporation of Cr from seawater into a shell during mineralization mediated by the organism. In particular, shells with higher δ53Cr values, which approach the Cr isotope composition of local seawater, tend to be associated with more negative Ce/Ce*. Importantly, the intercept of the above δ53Cr vs. Ce/Ce* correlation points to the Cr isotope composition of local ocean water, which has an average δ53Cr of +0.82±0.13‰ (2σ relative to SRM 979). These findings thus indicate that the above multi-proxy approach could be used to reconstruct the δ53Cr signature of local paleo-seawater based on Ce/Ce* and δ53Cr data in a set of well-preserved fossil skeletal carbonates (i.e., molluscan shells) collected at a specific site. Interestingly, the only calcifying organism from LEI that yielded identical δ53Cr vs. Ce/Ce* values as those in ambient ocean water was a microbial calcitic carbonate produced by red coralline algae (Lithothamnion sp.). This organism thus seems to incorporate Cr isotopes and REE from seawater without additional biological discrimination and/or isotope fractionation effects. Considering that calcite is a more stable CaCO3 polymorph during post-depositional alternation and diagenetic stabilization of marine carbonates (compared to aragonite), the fossil counterparts of these algal-microbial carbonates (microbialites) might thus represent ideal natural archives of the paleo-seawater δ53Cr and Ce/Ce* variations over geological time. Finally, our compilation of δ53Cr data from recent marine biogenic carbonates originating from the main oceanic provinces (South/North Pacific, South/North Atlantic, Caribbean, Mediterranean Sea) confirms that marine carbonates tend to be systematically enriched in light Cr isotopes relative to local ocean waters. Trace element constraints, however, indicate that some of these shifts to lower δ53Cr values (i.e., approaching −0.1 per mil) are related to a presence of lithogenic Cr in the shells, causing a diagenetic overprint of the primary marine δ53Cr signal.
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1016/j.epsl.2018.06.032
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