Central Tethyan platform-top hypoxia during Oceanic Anoxic Event 1a

Abstract

Abstract. Short-term hypoxia in epeiric water masses is a common phenomenon of modern marine environments and causes mass mortality in coastal marine ecosystems. Here, we test the hypothesis that during the early Aptian, platform-top hypoxia temporarily established in some of the vast epeiric seas of the central Tethys and caused, combined with other stressors, significant changes in reefal ecosystems. Potentially interesting target examples include time intervals characterized by the demise of lower Aptian rudist–coral communities and the establishment of microencruster facies, as previously described from the central and southern Tethys and from the proto-North Atlantic domain. These considerations are relevant as previous work has predominantly focused on early Aptian basinal anoxia in the context of Oceanic Anoxic Event (OAE) 1a, whereas the potential expansion of the oxygen minimum zone (OMZ) in coeval shallow-water environments is underexplored. Well-known patterns in the δ13C record during OAE 1a allow for a sufficiently time-resolved correlation with previously studied locations and assignment to chemostratigraphic segments. This paper presents and critically discusses the outcome of a multi-proxy study (e.g., rare earth elements (REEs), U isotopes, and redox-sensitive trace elements) applied to lower Aptian shallow-water carbonates today exposed in the Kanfanar quarry in Istria, Croatia. These rocks were deposited on an extensive, isolated high in the central Tethys surrounded by hemipelagic basins. Remarkably, during chemostratigraphic segment C2, the depletion of redox-sensitive trace elements As, V, Mo, and U in platform carbonates, deposited in normal marine oxic waters, record the first occurrence of basinal, organic-rich sediment deposition in which these elements are enriched. During the C3 segment, seawater oxygen depletion established on the platform top as indicated by the patterns in Ce/Ce* and U isotopes. Shifts in redox-sensitive proxies coincide with the expansion of microencruster facies. Segment C4 witnesses the return to normal marine reefal faunas on the platform top and is characterized by patterns in redox-sensitive proxies typical of normal marine dissolved oxygen levels. It remains unclear, however, if platform-top hypoxia resulted from the expansion and upwelling of basinal, oxygen-depleted water masses or if spatially isolated, shallow hypoxic water bodies formed on the platform. Data shown here are relevant as they shed light on the driving mechanisms that control poorly understood faunal patterns during OAE 1a in the neritic realm and provide evidence on the intricate relation between basinal and platform-top water masses.

Chemical oceanographic influences on sediment accumulations of a carbonate ramp: Holocene Yucatan Shelf, Mexico

“…In this formulation, a negative Ce anomaly, or a value increasingly less than one, suggests that Ce 4+ is less abundant (for example, due to oxidation); reducing conditions are suggested by a positive anomaly, or Ce/Ce* increasingly more than one. Of note, this formulation does not use Lanthanum in the calculation, so La enrichment would have no impact on calculated Ce/Ce* (Bau & Dulski, 1996;Lawrence et al, 2006;Hueter et al, 2019).…”

“…In many, or perhaps even most, situations in which a depositional signal can be isolated, such an approach is appropriate and reveals regional and even global dynamics (e.g. Bellanca et al, 1997;Zhao & Zheng, 2017;Li et al, 2017;Hueter et al, 2019;Liu et al, 2019).…”

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

“…2006, 2007; Hueter et al . 2019) and pelagic planktonic and benthic biota assemblages (Erba et al . 2010; Bottini et al .…”

“…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, but also 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 .…”