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.