Description: 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.

Description: Until recently it was assumed that the major modern ice sheets on Antarctica became established around the Eocene-Oligocene boundary about 34 Ma ago. But new evidence (e.g. Miller et al., 2008) indicates that continental ice may have been present much earlier, some of it probably even since the greenhouse times of the Late Cretaceous. Deep sea drilling data suggest changes in sea-level during the Late Cretaceous that could have been caused by the melting and freezing of vast ice sheets on Antarctica. Using a GCM approach to test the whether it would be possible to generate the described high-amplitude sealevel falls is one additional way to test this vigorously discussed issue. As shown above, our numerical approach indicates the possibility of a substantial Antarctic glaciation by changing the physical boundary conditions, eccentricity, pCO2, and elevation within reasonable Late Cretaceous ranges. Our simulations suggest that simulated snowfall and consecutive ice formation on Antarctica might yield sufficient volumes to account for the documented rapid, low-amplitude Cretaceous sea-level fluctuations. Based on cautious assumptions and possible errors the model results show that ice build-up could take place in realistic time spans and in accordance with the proxy records. Thus, the possibility of an Antarctic ice shield build-up large enough to drive sea level fluctuations on the order of tens of meters within 20,000-220,000 years is supported. The initial snow accumulation and following growth of Antarctic ice-sheets in the Cretaceous can be attributed to changes in southern hemisphere summer insolation due to reduced orbital eccentricity. Alternatively and/or additionally, declining atmospheric CO2 values caused further cooling

Description: Oceanic Anoxic Events (OAEs) document major perturbations of the global carbon cycle with repercussions on the Earth’s climate and ocean circulation that are relevant to understand future climate trends. Here, we compare sedimentation patterns, nutrient cycling, organic carbon accumulation and carbon isotope variability across Cretaceous Oceanic Anoxic Events OAE1a and OAE2 in two drill cores with unusually high sedimentation rates from the Vocontian Basin (southern France) and Tarfaya Basin (southern Morocco). OAE1a and OAE2 exhibit remarkable similarities in the evolution of their δ13C excursion with long-lasting negative carbon isotope excursions preceding the onset of both anoxic events, supporting the view that OAEs were triggered by massive emissions of volcanic CO2 into the atmosphere. Based on analysis of cyclic sediment variations, we estimated the duration of the individual phases within the carbon isotope excursions. For both events, we identify: (1) a precursor phase lasting ~ 430 kyr and ~ 130 kyr, (2) an onset phase of ~ 390 and ~ 70 kyr, (3) a peak phase of ~ 600 and ~ 90 kyr, (4) a plateau phase of ~ 1400 and ~ 200 kyr and (5) a recovery phase of ~ 630 and ~ 440 kyr, respectively. The total duration of the positive carbon isotope excursion is estimated as 3400 kyr and 790 kyr and that of the main carbon accumulation phase as 980 kyr and 180 kyr, for OAE1a and OAE 2 respectively. The extended duration of the peak, plateau and recovery phases requires fundamental changes in global nutrient cycles either (1) through excess nutrient inputs to the oceans by increasing continental weathering and river discharge or (2) through nutrient-recycling from the marine sediment reservoir. We investigated the role of phosphorus on the development of carbon accumulation by analysing phosphorus speciation across OAE2 and the mid-Cenomanian Event (MCE) in the Tarfaya Basin. The ratios of organic carbon and total nitrogen to reactive phosphorus (Corg/Preact and Ntotal/Preact) prior to OAE2 and the MCE hover close to or below the Redfield ratio characteristic of marine organic matter. Decreases in reactive phosphorus resulting in Corg/Preact and Ntotal/Preact above the Redfield ratio during the later phase of OAE2 and the MCE indicate leakage from the sedimentary column into the water column under the influence of intensified and expanded oxygen minimum zones. These results suggest that a positive feedback loop, rooted in the benthic phosphorus cycle, contributed to increased marine productivity and carbon burial over an extended period of time during OAEs.

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bretschneider, L., Hathorne, E. C., Huang, H., Luebbers, J., Kochhann, K. G. D., Holbourn, A., Kuhnt, W., Thiede, R., Gebregiorgis, D., Giosan, L., & Frank, M. Provenance and weathering of clays delivered to the Bay of Bengal during the middle Miocene: linkages to tectonics and monsoonal climate. Paleoceanography and Paleoclimatology, 36(2), (2021): e2020PA003917, https://doi.org/10.1029/2020PA003917.

Description: Tectonics and regional monsoon strength control weathering and erosion regimes of the watersheds feeding into the Bay of Bengal, which are important contributors to global climate evolution via carbon cycle feedbacks. The detailed mechanisms controlling the input of terrigenous clay to the Bay of Bengal on tectonic to orbital timescales are, however, not yet well understood. We produced orbital‐scale resolution geochemical records for International Ocean Discovery Program Site U1443 (southern Bay of Bengal) across five key climatic intervals of the middle to late Miocene (15.8–9.5 Ma). Our new radiogenic Sr, Nd, and Pb isotope time series of clays transported to the Ninetyeast Ridge suggest that the individual contributions from different erosional sources overall remained remarkably consistent during the Miocene despite major tectonic reorganizations in the Himalayas. On orbital timescales, however, high‐resolution data from the five investigated intervals show marked fluctuations of all three isotope systems. Interestingly, the variability was much higher within the Miocene Climatic Optimum (around 16–15 Ma) and across the major global cooling (~13.9–13.8 Ma) until ~13.5 Ma, than during younger time intervals. This change is attributed to a major restriction on the supply of High Himalayan erosion products due to migration of the peak precipitation area toward the frontal domains of the Himalayas and the Indo‐Burman Ranges. The transient excursions of the radiogenic isotope signals on orbital timescales most likely reflect climatically driven shifts in monsoon strength.

Description: This research used samples and data provided by the International Ocean Discovery Program and was funded by the German Research Foundation (DFG) (grants HA 5751/6‐1 and HA 5751/6‐2, KU 649/36‐1, and TH 1317‐8 and TH 1317‐9). Open access funding enabled and organized by Projekt DEAL.

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bretschneider, L., Hathorne, E. C., Bolton, C. T., Gebregiorgis, D., Giosan, L., Gray, E., Huang, H., Holbourn, A., Kuhnt, W., & Frank, M. Enhanced late miocene chemical weathering and altered precipitation patterns in the watersheds of the Bay of Bengal recorded by detrital clay radiogenic isotopes. Paleoceanography and Paleoclimatology, 36(9), (2021): e2021PA004252, https://doi.org/10.1029/2021PA004252.

Description: The late Miocene was a period of declining CO2 levels and extensive environmental changes, which likely had a large impact on monsoon strength as well as on the weathering and erosion intensity in the South Asian Monsoon domain. To improve our understanding of these feedback systems, detrital clays from the southern Bay of Bengal (International Ocean Discovery Program Site U1443) were analyzed for the radiogenic isotope compositions of Sr, Nd, and Pb to reconstruct changes in sediment provenance and weathering regime related to South Asian Monsoon rainfall from 9 to 5 Ma. The 100 kyr resolution late Miocene to earliest Pliocene record suggests overall low variability in the provenance of clays deposited on the Ninetyeast Ridge. However, at 7.3 Ma, Nd and Pb isotope compositions indicate a switch to an increased relative contribution from the Irrawaddy River (by ∼10%). This shift occurred during the global benthic δ13C decline, and we suggest that global cooling and increasing aridity resulted in an eastward shift of precipitation patterns leading to a more focused erosion of the Indo-Burman Ranges. Sr isotope compositions were decoupled from Nd and Pb isotope signatures and became more radiogenic between 6 and 5 Ma. Grassland expansion generating thick, easily weatherable soils may have led to an environment supporting intense chemical weathering, which is likely responsible for the elevated detrital clay 87Sr/86Sr ratios during this time. This change in Sr isotope signatures may also have contributed to the late Miocene increase of the global seawater Sr isotope composition.

Description: This research used samples and data provided by the International Ocean Discovery Program and was funded by the German Research Foundation (DFG) (grants HA 5751/6-1 & -2). C. T. Bolton acknowledges funding from the French ANR project iMonsoon (ANR-16-CE01-0004-01) and IODP France. W. Kuhnt acknowledges funding from the DFG (grant Ku649/36-1).