Tropical temperature in the Maastrichtian Danish Basin: Data from coccolith Δ47 and δ18O

Abstract

The Cretaceous Earth, with its greenhouse climate and absence of major ice caps in the polar regions, represents an extreme scenario for modeling future warming. Despite considerable efforts, we are just at the verge of fully understanding the conditions of a warm Earth, and better, more extensive proxy evidence is needed to solve existing discrepancies between the applied temperature proxies. In particular, the Maastrichtian temperature trends are controversial, since data indicate cooling in the South Atlantic and contemporary warming of the North Atlantic. The “heat piracy” hypothesis involves northward heat transport to midlatitudes via oceanic currents and is used to explain the contrasting polar cooling/warming patterns. Here, we present Δ47 and δ18O data from nine coccolith-enriched samples from a shallow core taken from the Danish Basin (Chalk Sea), representing a key location at the northern mid-latitudes. Based on Δ47 data of coccolith-enriched material, sea-surface temperatures for the late Campanian–Maastrichtian ranged from 24 °C to 30 °C, with an average of 25.9 °C ± 2 °C. This is 4–6 °C higher than estimates based on Δ47 of bulk samples and 8–10 °C higher than reported temperatures based on bulk δ18O data from the same core. However, these higher temperature estimates are lower, but overall in line with estimates of Late Cretaceous tropical sea-surface temperatures from TEX86 (tetraether index of 86 carbons), when considering latitudinal differences. The study highlights the potential of clumped isotope paleothermometry on coccoliths as a valid, reliable proxy with which to reconstruct sea-surface temperatures.

Shell chemistry of the boreal Campanian bivalve <i>Rastellum diluvianum</i> (Linnaeus, 1767) reveals temperature seasonality, growth rates and life cycle of an extinct Cretaceous oyster

“…If local δ 18 O sw values at Ivö Klack were indeed 1 ‰-1.5 ‰ reduced with respect to those in the fully marine Boreal Chalk Sea, and marine δ 18 O sw was around 0-0.5 ‰ VS-MOW rather than the assumed −1 ‰ VSMOW, the effects of these two biases cancel each other out, and the best estimation of the extreme seasonal SST range at Ivö Klack based on this study's data would be 14-25 • C with a MAT of 19 • C. This MAT is comparable to the MAT of the late early Campanian Boreal Chalk Sea waters of 17-19 • C calculated from coccolith δ 18 O c (Lowenstam and Epstein, 1954;Jenkyns et al, 2004;Friedrich et al, 2005;Thibault et al, 2016) and slightly warmer than mean annual air temperatures from this paleolatitude based on phosphate-δ 18 O reconstructions (±15 • C; Amiot et al, 2004). However, Ivö Klack SSTs are ∼ 6 • C colder than the clumped isotope-based reconstructions from marine chalk samples (Tagliavento et al, 2019). The latter could indicate that coastal SSTs and air temperatures were much colder than marine temperatures in the Campanian higher latitudes, but such temperature differences are highly unusual compared to modern climates.…”

“…Considering such a deviation would reduce reconstructed temperatures by 4-6 to 10-21 • C, much colder than open marine reconstructions of the Boreal Chalk Sea by Thibault et al (2016). This result would be in strong disagreement with a recent study by Tagliavento et al (2019) in which clumped isotope analyses (which do not rely on the assumption of constant δ 18 O sw ) were used to correct the δ 18 O c -based reconstructions of the boreal chalk and yielded higher temperatures (∼ 26 • C MAT for open marine SST) and a correction of δ 18 O sw towards 1 ‰-1.5 ‰ heavier values (resulting in a Campanian δ 18 O sw of −0.5 ‰-0 ‰ VSMOW). Another caveat is that salinity effects on local δ 18 O sw strongly depend on the local δ 18 O sw of the local freshwater source (riverine or precipitation), which in the present-day higher mid-latitudes is around −7 ‰ VS-MOW to −8 ‰ VSMOW (e.g., Ullmann et al, 2010), but this is impossible to constrain at Ivö Klack during the Campanian within the scope of this study.…”

“…The latter could indicate that coastal SSTs and air temperatures were much colder than marine temperatures in the Campanian higher latitudes, but such temperature differences are highly unusual compared to modern climates. Alternatively, this difference could highlight a severe temperature bias in (both phosphate and carbonate) δ 18 O-based reconstructions, which should be further investigated using independent marine temperature proxies such as the clumped isotope paleothermometer (e.g., de Winter et al, 2018;Tagliavento et al, 2019).…”

“…These chalk successions are featured in various paleoclimate studies because they are accessible in good outcrops and consist predominantly of calcareous nannofossils which were thought to faithfully record sea surface conditions (e.g., Jenkyns et al, 1994). However, recent studies (e.g., O'Brien et al, 2017;Tagliavento et al, 2019) have shown that diagenetic overprinting likely biases these records towards cooler temperatures, resulting in the apparent cool tropics paradox (Pearson et al, 2001). Even though sea level changes seem to indicate possible small changes in land-ice volume during the Late Cretaceous, warm high-latitude paleotemperatures likely rule out the possibility of extensive polar ice sheets comparable in volume to modern ice caps (Barrera and Johnson, 1999;Huber et al, 2002;Jenkyns et al, 2004;Miller et al, 2005;Thibault et al, 2016).…”

First absolute seasonal temperature estimates for greenhouse climate from clumped isotopes in bivalve shells

“…3A). A cold bias of ~8°C in δ 18 Oc-based SST reconstructions due to inaccurate δ 18 Osw assumptions was previously documented through the Δ47 method and proxy comparison 8,27 , however this does not account for seasonal biases in the proxies.…”