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Neodymium isotopic evidence for large-scale oceanographic change during the collapse of the Cretaceous hothouse

Abstract : After the peak hothouse conditions of the Late Cretaceous, ending at ~91 Ma, climate was characterised by a gradual decrease in temperatures and CO2 levels, an absence of major carbon cycle perturbations, and a reorganisation of deep-water circulation patterns. Throughout this time, the Atlantic Ocean was gradually opening and sea level was higher than at present, with large parts of north-western Europe covered by shallow epicontinental chalk seas. The role of surface-water oceanography in the long-term Late Cretaceous climatic cooling is poorly understood, as reconstructed upper-ocean circulation patterns are based on relatively low-resolution records that have often been assembled from multiple localities. Here we present a ~28 Myr continuous record of neodymium-isotope ratios (εNd) of fish debris from the Trunch borehole of Norfolk, England, to reconstruct the evolution of upper ocean waters of the Boreal-Tethyan epicontinental shelf during the Late Cretaceous. During the Cenomanian–Turonian, background εNd values are in the range of -9 to -10, comparable to previously published high-resolution datasets from the elsewhere in southern England that span Oceanic Anoxic Event 2 (OAE 2). Unfortunately, OAE 2 is marked by a disconformity in the Trunch core. Surprisingly, our record shows a ~5 unit positive excursion during the mid–late Turonian, a much larger shift than those recorded in southern England during OAE 2. The εNd excursion lasts ~1.5 Myrs, and coincides with cooling observed in oxygen-isotope and faunal records across the Chalk Sea, a global positive δ13C excursion, and a major change in sea-level, suggesting a potentially global driver of climate- and circulation change. The high εNd values (peaking at -5.9 units) indicate basalt–seawater interactions, probably in the Boreal Sea, suggesting that volcanic activity and/or basalt weathering accompanied the cooling. After the late Turonian, Nd-isotope values return to relatively steady background levels of -11 to -12 in the Coniacian–Campanian; this long-term stability of circulation in the Chalk Sea suggests that circulation in this region was neither driving nor responding to the long-term global cooling trend. Further, the strongly unradiogenic signature of the Trunch record suggests a decline in influence from other water masses—Boreal or Tethyan—consistent with a restriction of low-latitude Pacific–Tethyan gateways. Our εNd data, particularly the unexpected Nd-isotope variability in the Turonian, highlight the necessity to look beyond abrupt climate perturbations and to generate long-term continuous proxy records to gain a thorough understanding of climate processes in a greenhouse world.
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Submitted on : Monday, October 11, 2021 - 2:49:22 PM
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  • HAL Id : insu-03373434, version 1

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Sietske J. Batenburg, Eleanor Drage, Elaine Yi Gao, Lauren O'Connor, Hugh C. Jenkyns, et al.. Neodymium isotopic evidence for large-scale oceanographic change during the collapse of the Cretaceous hothouse. 35th IAS Meeting of Sedimentology, Jun 2021, Virtual, Czech Republic. pp.46. ⟨insu-03373434⟩

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