TY - JOUR

T1 - Evaluating the role of coastal hypoxia on the transient expansion of microencruster intervals during the early Aptian

AU - Hueter, Alexander

AU - Huck, Stefan

AU - Heimhofer, Ulrich

AU - Bodin, Stéphane

AU - Weyer, Stefan

AU - Jochum, Klaus P.

AU - Roebbert, Yvonne

AU - Immenhauser, Adrian

N1 - Funding Information: We thank Nadja Pierau, Annika Neddermeyer and Lena Steinmann (Institute of Geology, Leibniz University Hannover, Germany) for their advice in uranium geochemistry and support in the laboratory. Special thanks to Gerald H. Haug (Max Planck Institute for Chemistry, Mainz, Germany) for the opportunity to perform LA‐ICP‐MS measurements. The authors would like to thank Brigitte Stoll and Ulrike Weis (Max Planck Institute for Chemistry, Mainz, Germany) for preparing the LA‐ICP‐MS measurements. This project was funded by the German Science Foundation (DFG, Project IM44/19‐1 and HU2258/3‐1). The authors declare that they have no conflict of interest. Open access funding enabled and organized by Projekt DEAL.

PY - 2021/6/11

Y1 - 2021/6/11

N2 - Worldwide, a growing number of modern coastal marine ecosystems are increasingly exposed to suboxic- or even anoxic conditions. Low seawater oxygen levels trigger significant ecosystem changes and may result in mass mortality of oxygen-sensitive biota. The applicability of observations from recent (anthropogenically influenced) suboxic coastal settings to fossil anoxic shallow-marine environments is, however, as yet poorly explored. The test case documented here are upper Barremian to lower Aptian strata in the Lusitanian Basin (Ericeira section, Portugal). These are characterized by the transient demise of rudist–coral communities and the rapid establishment of microencruster facies in the vacant ecological niches. The hypothesis is tested that the temporal expansion of the microencrusting organism Lithocodium aggregatum took place in response to platform-top seawater oxygen depletion. We critically discuss the outcome of a multi-proxy palaeoseawater redox approach (e.g. Rare Earth Elements (REEs), U isotopes and palaeoecology) and put the robustness of the proxies applied here to the test. This is done by considering issues with these methods in general but also emphasizing the significance of terrigenous contamination and fractionation effects. Data shown here document that evidence for coastal seawater oxygen depletion in the prelude of Oceanic Anoxic Event (OAE) 1a is lacking, and hence, anoxia was not the driving mechanism for the demise of rudist–coral ecosystems in the proto-North Atlantic platform setting studied here. In contrast, well-oxygenated early Aptian platform-top water masses are proposed for this site. Geologically short (decades to millennia) fluctuations in seawater oxygen levels cannot be excluded, however. But even if these took place, they offer no explanation for the Kyr to Myr-scale patterns discussed here. The present paper is relevant as it sheds light on the complexity of mechanisms that drive punctuated Early Cretaceous coral–rudist ecosystem turnover, and assess strengths and weaknesses of redox proxies applied to ancient shallow-marine platform carbonates.

AB - Worldwide, a growing number of modern coastal marine ecosystems are increasingly exposed to suboxic- or even anoxic conditions. Low seawater oxygen levels trigger significant ecosystem changes and may result in mass mortality of oxygen-sensitive biota. The applicability of observations from recent (anthropogenically influenced) suboxic coastal settings to fossil anoxic shallow-marine environments is, however, as yet poorly explored. The test case documented here are upper Barremian to lower Aptian strata in the Lusitanian Basin (Ericeira section, Portugal). These are characterized by the transient demise of rudist–coral communities and the rapid establishment of microencruster facies in the vacant ecological niches. The hypothesis is tested that the temporal expansion of the microencrusting organism Lithocodium aggregatum took place in response to platform-top seawater oxygen depletion. We critically discuss the outcome of a multi-proxy palaeoseawater redox approach (e.g. Rare Earth Elements (REEs), U isotopes and palaeoecology) and put the robustness of the proxies applied here to the test. This is done by considering issues with these methods in general but also emphasizing the significance of terrigenous contamination and fractionation effects. Data shown here document that evidence for coastal seawater oxygen depletion in the prelude of Oceanic Anoxic Event (OAE) 1a is lacking, and hence, anoxia was not the driving mechanism for the demise of rudist–coral ecosystems in the proto-North Atlantic platform setting studied here. In contrast, well-oxygenated early Aptian platform-top water masses are proposed for this site. Geologically short (decades to millennia) fluctuations in seawater oxygen levels cannot be excluded, however. But even if these took place, they offer no explanation for the Kyr to Myr-scale patterns discussed here. The present paper is relevant as it sheds light on the complexity of mechanisms that drive punctuated Early Cretaceous coral–rudist ecosystem turnover, and assess strengths and weaknesses of redox proxies applied to ancient shallow-marine platform carbonates.

KW - Anoxia

KW - cerium anomalies

KW - Cretaceous

KW - Oceanic Anoxic Event 1a

KW - redox proxies

KW - uranium isotopes

UR - http://www.scopus.com/inward/record.url?scp=85096803271&partnerID=8YFLogxK

U2 - 10.1111/let.12411

DO - 10.1111/let.12411

M3 - Article

AN - SCOPUS:85096803271

VL - 54

SP - 399

EP - 418

JO - LETHAIA

JF - LETHAIA

SN - 0024-1164

IS - 3

ER -