Records of past mid-depth ventilation: Cretaceous ocean anoxic event 2 vs. Recent oxygen minimum zones

摘要

Present day oceans are well ventilated, with the exception of mid-depth oxygen minimum zones (OMZs)under high surface water productivity, regions of sluggishcirculation, and restricted marginal basins. In the Mesozoic, however,entire oceanic basins transiently became dysoxic or anoxic. The Cretaceousocean anoxic events (OAEs) were characterised by laminated organic-carbonrich shales and low-oxygen indicating trace fossils preserved in thesedimentary record. Yet assessments of the intensity and extent ofCretaceous near-bottom water oxygenation have been hampered by deep orlong-term diagenesis and the evolution of marine biota serving as oxygenindicators in today's ocean. Sedimentary features similar to those found inCretaceous strata were observed in deposits underlying Recent OMZs, wherebottom-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 thePeruvian upwelling with deposits of the late Cenomanian OAE 2 from the north-westAfrican shelf. Holocene laminated sediments are encountered at bottom-wateroxygen levels of < 7 μmol kg?1 under the Peruvianupwelling and < 5 μmol kg?1 in California Borderlandbasins and the Pakistan Margin. Seasonal to decadal changes of sedimentinput are necessary to create laminae of different composition. However,bottom currents may shape similar textures that are difficult to discernfrom primary seasonal laminae. The millimetre-sized trace fossilChondrites was commonly found in Cretaceous strata and Recent oxygen-depletedenvironments where its diameter increased with oxygen levels from 5 to 45 μmol kg?1. Chondrites has not been reported in Peruvian sediments butcentimetre-sized crab burrows appeared around 10 μmol kg?1, which mayindicate a minimum oxygen value for bioturbated Cretaceous strata. Organiccarbon 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 lateHolocene accumulation rates of laminated Peruvian sediments under Recentoxygen levels below 5 μmol kg?1. Sediments deposited at> 10 μmol kg?1 showed an inverse exponentialrelationship of bottom-water oxygen levels and organic carbon accumulationdepicting enhanced bioirrigation and decomposition of organic matter withincreased oxygen supply. In the absence of seasonal laminations and underconditions of low burial diagenesis, this relationship may facilitatequantitative estimates of palaeo-oxygenation. Similarities and differencesbetween Cretaceous OAEs and late Quaternary OMZs have to be further exploredto improve our understanding of sedimentary systems under hypoxicconditions.