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

Alexander Hueter,Adrian Immenhauser,Stefan Weyer,Yvonne Roebbert,Klaus P. Jochum,Ulrich Heimhofer,Stéphane Bodin,Stefan Huck

doi:10.1111/let.12411

Abstract

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.

Highlights

Oxygen is a key element for the metabolism of most marine organisms
Following previous workers (Leinfelder et al 1993; Rameil et al 2010), we argue that these microencruster communities, similar to many modern marine organisms (Diaz & Rosenberg 1995; Altieri & Diaz 2019), were significantly more tolerant with regard to environmental stressors including low dissolved seawater oxygen levels compared to coral– rudist ecosystems
In order to simplify the description, the succession is divided into three main stratigraphical intervals with different facies associations: (1) lower normal marine facies (0 to 2.5 m); (2) L. aggregatum facies (2.5 to 6.1 m); and (3) upper normal marine facies (6.1 to 8.5 m)

Summary

Introduction

Oxygen is a key element for the metabolism of most marine organisms. Seawater oxygen depletion is a threat, to shallow coastal areas, and the number of seasonal to permanent oxygen‐depleted coasts has dramatically increased during the past 50 years (Diaz & Rosenberg 2008). The approach followed here applies tools and concepts recently tested in the Central Tethyan realm (Kanfanar section, Croatia; Hueter et al 2019) These tools include detailed fieldwork, carbonate microfacies analysis and SEM analysis, and involve state‐ of‐the‐art geochemical seawater redox proxies such as cerium (Ce) anomalies and uranium (U) isotope ratios. There, these events are coeval with OAE 1a and related organic‐rich pelagic deposits (black shales) documenting basinal anoxia (Hueter et al 2019) This is of relevance for the proto‐North Atlantic case example tested here, as lower Aptian shallow‐water limestones of the Lusitanian Basin (Portugal) were deposited prior to the onset of OAE 1a (Heimhofer et al 2007; Burla et al 2008; Huck et al 2012) but show microencruster intervals (correlated between the Ericeira, Cresmina and Sao Juliao sections) comparable to those in the Central Tethyan realm

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Conclusion