Abstract
Abstract. Present day oceans are well ventilated, with the exception of mid-depth oxygen minimum zones (OMZs) under high surface water productivity, regions of sluggish circulation, and restricted marginal basins. In the Mesozoic, however, entire oceanic basins transiently became dysoxic or anoxic. The Cretaceous ocean anoxic events (OAEs) were characterised by laminated organic-carbon rich shales and low-oxygen indicating trace fossils preserved in the sedimentary record. Yet assessments of the intensity and extent of Cretaceous near-bottom water oxygenation have been hampered by deep or long-term diagenesis and the evolution of marine biota serving as oxygen indicators in today's ocean. Sedimentary features similar to those found in Cretaceous strata were observed in deposits underlying Recent OMZs, where bottom-water oxygen levels, the flux of organic matter, and benthic life have been studied thoroughly. Their implications for constraining past bottom-water oxygenation are addressed in this review. We compared OMZ sediments from the Peruvian upwelling with deposits of the late Cenomanian OAE 2 from the north-west African shelf. Holocene laminated sediments are encountered at bottom-water oxygen levels of < 7 μmol kg−1 under the Peruvian upwelling and < 5 μmol kg−1 in California Borderland basins and the Pakistan Margin. Seasonal to decadal changes of sediment input are necessary to create laminae of different composition. However, bottom currents may shape similar textures that are difficult to discern from primary seasonal laminae. The millimetre-sized trace fossil Chondrites was commonly found in Cretaceous strata and Recent oxygen-depleted environments where its diameter increased with oxygen levels from 5 to 45 μmol kg−1. Chondrites has not been reported in Peruvian sediments but centimetre-sized crab burrows appeared around 10 μmol kg−1, which may indicate a minimum oxygen value for bioturbated Cretaceous strata. Organic carbon accumulation rates ranged from 0.7 and 2.8 g C cm−2 kyr−1 in laminated OAE 2 sections in Tarfaya Basin, Morocco, matching late Holocene accumulation rates of laminated Peruvian sediments under Recent oxygen levels below 5 μmol kg−1. Sediments deposited at > 10 μmol kg−1 showed an inverse exponential relationship of bottom-water oxygen levels and organic carbon accumulation depicting enhanced bioirrigation and decomposition of organic matter with increased oxygen supply. In the absence of seasonal laminations and under conditions of low burial diagenesis, this relationship may facilitate quantitative estimates of palaeo-oxygenation. Similarities and differences between Cretaceous OAEs and late Quaternary OMZs have to be further explored to improve our understanding of sedimentary systems under hypoxic conditions.
Highlights
In the present day ocean, most of the water column is well ventilated as a consequence of thermohaline circulation processes that lead to subduction of cold, oxygen rich and dense water masses in high northern and southern latitudes (e.g. Kuhlbrodt et al, 2007)
The Pleistocene to Holocene and late Cenomanian to early Turonian stages are more than 94 million years apart in Earth’s history
A direct comparison of their sedimentary record and environmental processes is hampered by burial diagenesis, evolution of marine biota, different continental and ocean configurations, and different climates, ocean circulation, and biogeochemical cycles
Summary
In the present day ocean, most of the water column is well ventilated as a consequence of thermohaline circulation processes that lead to subduction of cold, oxygen rich and dense water masses in high northern and southern latitudes (e.g. Kuhlbrodt et al, 2007). In the present day ocean, most of the water column is well ventilated as a consequence of thermohaline circulation processes that lead to subduction of cold, oxygen rich and dense water masses in high northern and southern latitudes Regional or global ventilation of the ocean underwent significant changes on different time scales due to a variety of reasons, including changes in atmospheric and ocean circulation, stratification, temperature or tectonic processes. It is, difficult to quantify the past spatial extent and intensity of oxygen minima because the oxygen concentration of the water column is not directly recorded in the sediments. Derivative proxies have been applied to reconstruct past ocean oxygenation
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