Mesozoic climates and oceans – a tribute to Hugh Jenkyns and Helmut Weissert

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.The study of past greenhouse climate intervals in Earth history, such as the Mesozoic, is an important, relevant and dynamic area of research for many sedimentary geologists, geochemists, palaeontologists and climate modellers. The Mesozoic sedimentary record provides key insights into the mechanics of how the Earth system works under warmer conditions, providing examples of natural climate change and perturbations to ocean chemistry, including anoxia, that are of societal relevance for understanding and contextualizing ongoing and future environmental problems. Furthermore, the deposition of widespread organic-carbon-rich sediments (‘black shales’) during the Mesozoic means that this is an era of considerable economic interest. In July 2015, an international group of geoscientists attended a workshop in Ascona, Switzerland, to discuss all aspects of the Mesozoic world and to celebrate the four-decade-long contributions made by Hugh Jenkyns (University of Oxford) and Helmut Weissert (ETH Zürich) to our understanding of this fascinating era in Earth history. This volume of Sedimentology arose from that meeting and contains papers inspired by (and co-authored by!) Hugh and Helmi. Here, a brief introduction to the volume is provided that reviews aspects of Hugh and Helmi's major achievements; contextualizes the papers of the Thematic Issue; and discusses some of the outstanding questions and areas for future research

An impulse response function for the "long tail" of excess atmospheric CO₂ in an Earth system model

The ultimate fate of (fossil fuel) CO emitted to the atmosphere is governed by a range of sedimentological and geological processes operating on timescales of up to the ca. hundred thousand year response of the silicate weathering feedback. However, how the various geological CO sinks might saturate and feedbacks weaken in response to increasing total emissions is poorly known. Here we explore the relative importance and timescales of these processes using a 3-D ocean-based Earth system model. We first generate an ensemble of 1 Myr duration CO decay curves spanning cumulative emissions of up to 20,000 Pg C. To aid characterization and understanding of the model response to increasing emission size, we then generate an impulse response function description for the long-term fate of CO in the model. In terms of the process of carbonate weathering and burial, our analysis is consistent with a progressively increasing fraction of total emissions that are removed from the atmosphere as emissions increase, due to the ocean carbon sink becoming saturated, together with a lengthening of the timescale of removal from the atmosphere. However, we find that in our model the ultimate CO sink - silicate weathering feedback - is approximately invariant with respect to cumulative emissions, both in terms of its importance (it removes the remaining excess ~7% of total emissions from the atmosphere) and timescale (~270 kyr). Because a simple pulse-response description leads to initially large predictive errors for a realistic time-varying carbon release, we also develop a convolution-based description of atmospheric CO decay which can be used as a simple and efficient means of making long-term carbon cycle perturbation projections. Key Points An ensemble of CO pulse emissions are modeled using an Earth system model Our impulse response function projects the atmospheric lifetime of emitted CO We characterize how the marine CO sinks tend to saturate at very high emissions. 2 2 2 2 2 2 2 2

Upper ocean oxygenation dynamics from I/Ca ratios during the Cenomanian-Turonian OAE 2

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 30 (2015): 510–526, doi:10.1002/2014PA002741.Global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans, leading to increases in organic production, the degradation of which causes a further decrease in dissolved oxygen. In extreme cases in the geological past, this trajectory has led to catastrophic marine oxygen depletion during the so-called oceanic anoxic events (OAEs). How the water column oscillated between generally oxic conditions and local/global anoxia remains a challenging question, exacerbated by a lack of sensitive redox proxies, especially for the suboxic window. To address this problem, we use bulk carbonate I/Ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the Cretaceous OAE 2. In general, I/Ca ratios were relatively low preceding and during the OAE interval, indicating deep suboxic or anoxic waters exchanging directly with near-surface waters. However, individual sites display a wide range of initial values and excursions in I/Ca through the OAE interval, reflecting the importance of local controls and suggesting a high spatial variability in redox state. Both I/Ca and an Earth System Model suggest that the northeast proto-Atlantic had notably higher oxygen levels in the upper water column than the rest of the North Atlantic, indicating that anoxia was not global during OAE 2 and that important regional differences in redox conditions existed. A lack of correlation with calcium, lithium, and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the I/Ca proxy during OAE 2.Z.L. thanks NSF OCE 1232620. J.D.O. is supported by an Agouron Postdoctoral Fellowship. T.W.L. acknowledges support from the NSF-EAR and NASA-NAI. A.R. thanks the support of NERC via NE/J01043X/1.2015-11-1

Redox‐controlled preservation of organic matter during “OAE 3” within the Western Interior Seaway

During the Cretaceous, widespread black shale deposition occurred during a series of Oceanic Anoxic Events (OAEs). Multiple processes are known to control the deposition of marine black shales, including changes in primary productivity, organic matter preservation, and dilution. OAEs offer an opportunity to evaluate the relative roles of these forcing factors. The youngest of these events—the Coniacian to Santonian OAE 3—resulted in a prolonged organic carbon burial event in shallow and restricted marine environments including the Western Interior Seaway. New high‐resolution isotope, organic, and trace metal records from the latest Turonian to early Santonian Niobrara Formation are used to characterize the amount and composition of organic matter preserved, as well as the geochemical conditions under which it accumulated. Redox sensitive metals (Mo, Mn, and Re) indicate a gradual drawdown of oxygen leading into the abrupt onset of organic carbon‐rich (up to 8%) deposition. High Hydrogen Indices (HI) and organic carbon to total nitrogen ratios (C:N) demonstrate that the elemental composition of preserved marine organic matter is distinct under different redox conditions. Local changes in δ13C indicate that redox‐controlled early diagenesis can also significantly alter δ13Corg records. These results demonstrate that the development of anoxia is of primary importance in triggering the prolonged carbon burial in the Niobrara Formation. Sea level reconstructions, δ18O results, and Mo/total organic carbon ratios suggest that stratification and enhanced bottom water restriction caused the drawdown of bottom water oxygen. Increased nutrients from benthic regeneration and/or continental runoff may have sustained primary productivity.Key PointsBottom water redox changes triggered carbon burial within the WIS during OAE 3Anoxia developed due to O2 drawdown in a stratified water columnRedox‐controlled changes in OM preservation altered primary δ13Corg signalsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112294/1/palo20210.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112294/2/palo20210-sup-0001-SupportingInfo.pd

An extraterrestrial trigger for the Early Cretaceous massive volcanism? Evidence from the paleo-Tethys Ocean

The Early Cretaceous Greater Ontong Java Event in the Pacific Ocean may have covered ca. 1% of the Earth's surface with volcanism. It has puzzled scientists trying to explain its origin by several mechanisms possible on Earth, leading others to propose an extraterrestrial trigger to explain this event. A large oceanic extraterrestrial impact causing such voluminous volcanism may have traces of its distal ejecta in sedimentary rocks around the basin, including the paleo-Tethys Ocean which was then contiguous with the Pacific Ocean. The contemporaneous marine sequence at central Italy, containing the sedimentary expression of a global oceanic anoxic event (OAE1a), may have recorded such ocurrence as indicated by two stratigraphic intervals with 187Os/188Os indicative of meteoritic influence. Here we show, for the first time, that platinum group element abundances and inter-element ratios in this paleo-Tethyan marine sequence provide no evidence for an extraterrestrial trigger for the Early Cretaceous massive volcanism

Midlatitude shelf seas in the Cenomanian-Turonian greenhouse world: Temperature evolution and North Atlantic circulation

An 8 million year record of subtropical and midlatitude shelf-sea temperatures, derived from oxygen isotopes of well-preserved brachiopods from a variety of European sections, demonstrates a long-term Cenomanian temperature rise (16–20°C, midlatitudes) that reached its maximum early in the late Turonian (23°C, midlatitudes). Superimposed on the long-term trend, shelf-sea temperatures vary at shorter timescales in relation to global carbon cycle perturbations. In the mid-Cenomanian and the late Turonian, two minor shelf-sea cooling events (2–3°C) coincide with carbon cycle perturbations and times of high-amplitude sea level falls. Although this evidence supports the hypothesis of potential glacioeustatic effects on Cretaceous sea level, the occurrence of minimum shelf-sea temperatures within transgressive beds argues for regional changes in shelf-sea circulation as the most plausible mechanism. The major carbon cycle event in the latest Cenomanian (oceanic anoxic event 2) is accompanied by a substantial increase in shelf-sea temperatures (4–5°C) that occurred ∼150 kyr after the commencement of the δ13C excursion and is related to the spread of oceanic conditions in western European shelf-sea basins. Our oxygen isotope record and published δ18O data of pristinely preserved foraminifera allow the consideration of North Atlantic surface water properties in the Cenomanian along a transect from the tropics to the midlatitudes. On the basis of fossil-derived δ18O, estimated δw ranges, and modeled salinities, temperature-salinity-density ranges were estimated for tropical, subtropical, and midlatitude surface waters. Accordingly, the Cenomanian temperate shelf-seas waters have potentially the highest surface water density and could have contributed to North Atlantic intermediate to deep waters in the preopening stage of the equatorial Atlantic gateway