Late Cretaceous seasonal ocean variability from the Arctic

Abstract

The modern Arctic Ocean is regarded as barometer of global change and amplifier of global warming 1 and therefore records of past Arctic change are of a premium for palaeoclimate reconstruction. Little is known of the state of the Arctic Ocean in the greenhouse period of the late Cretaceous, yet records from such times may yield important clues to its future behaviour given current global warming trends. Here we present the first seasonally resolved sedimentary record from the Cretaceous from the Alpha Ridge of the Arctic Ocean. This "paleo-sediment trap" provides new insights into the workings of the Cretaceous marine biological carbon pump. Seasonal primary production was dominated by diatom algae but was not related to upwelling as previously hypothesised 2 . Rather, production occurred within a stratified water column, involving specially adapted species in blooms resembling those of the modern North Pacific Subtropical Gyre 3 , or those indicated for the Mediterranean sapropels 4 . With elevated CO 2 levels and warming currently driving increased stratification in the global ocean 5 this stratified-adapted style of production may become more widespread.Our evidence for seasonal diatom production and flux testify to an ice free summer, but thin accumulations of terrigenous sediment within the diatom ooze are consistent with the presence of intermittent sea ice in the winter, supporting a wide body of evidence for low Arctic late Cretaceous winter temperatures 6-8 rather than recent suggestions of a 15°C mean annual temperature at this time 9 . 3The Arctic is a critical yet under-sampled region for palaeoclimate studies. The recent ACEX coring has provided Arctic records back to the Palaeogene 10 but earlier shallow coring of older, Cretaceous sediments has hitherto offered only tantalising indications of the Arctic palaeoenvironment 2,9,11,12 . Such sediments afford the opportunity to investigate Arctic climate variability in past greenhouse states that may be analogues for the future. Specifically, little is known of Arctic seasonal-scale climate variability in periods without permanent sea ice cover. Cretaceous laminated sediments also provide a "palaeo-sediment trap" record whereby the past workings of the marine biological carbon pump may be elucidated. In the modern ocean diatom algae are responsible for up to 40% of oceanic primary production and because they dominate export in many marine environments, diatoms are the key agents in the marine biological carbon pump, central to biogeochemical cycling 13,14 . By contrast, the role of diatoms in the Cretaceous oceans is poorly understood, in part, due to lack of preservation, since opal A is unstable and diatoms are easily destroyed during sediment burial and silica diagenesis. However, the first, albeit rare, pelagic diatomites occur in the late Cretaceous coinciding with a radiation of planktonic diatoms 15 , and in late Cretaceous sediments without surviving biosilica, there is increasing biomarker evidence of diatom contribution to carbon cyc...

Seismic velocities within the sedimentary succession of the Canada Basin and southern Alpha-Mendeleev Ridge, Arctic Ocean: evidence for accelerated porosity reduction?

“…Geophysical constraints (Sweeney & Weber 1986), and radiometric dating of a single basaltic sample , suggest that the complex was largely constructed in the Early Cretaceous with late phases of magmatism persisting into the Late Cretaceous. Evidence from shallow core samples indicates that it had subsided and was undergoing burial by the late Campanian or early Maastrictian (Mudie & Blasco 1985;Davies et al 2009). …”

Trochosira polychaeta: a colonial diatom from the late Cretaceous exhibiting two contrasting chain-linking mechanisms and a wide range of preservation

“…The preservation of delicate and lightly silicified parts of the frustule affords an excellent opportunity to make the distinction between resting spores and vegetative cells and to document mechanisms of chain formation. Furthermore, the laminated sediments of the CESAR--6 core represent a palaeo--sediment trap that preserves successive flux events permitting the identification of co--sedimented diatoms from which more robust ecological interpretations may be made of associated species (Davies et al, 2009). In this contribution we document one species, Trochosira polychaeta Strelnikova that has been assigned resting spore status by some earlier studies (Kitchell et al, 1986;Dell'Agnese & Clark, 1994).…”

“…Sediment samples were taken from the Alpha Ridge CESAR--6 core, interval 134 -295 cm, held at the Bedford Institute of Oceanography (see Davies et al 2009 for detailed core description and stratigraphy). On sampling, the perfectly curated archive half was still wet and resembled a near--surface modern core of deep--sea diatom ooze.…”

“…An exception to this is the shallow buried Cretaceous sediment (CESAR 6 and Fl--437 cores) from the Alpha Ridge of the Arctic Ocean, that contains superbly preserved diatomaceous material (Barron, 1985;Dell'agnese & Clark, 1994;Davies et al, 2009). The preservation of delicate and lightly silicified parts of the frustule affords an excellent opportunity to make the distinction between resting spores and vegetative cells and to document mechanisms of chain formation.…”

Yukon Taiga – Past, Present and Future

“…When the Atlantic Ocean began forming and distancing Eurasian and North American continents, ~ 170 Ma, they were approaching their present-day latitudes and shapes. Although Late Cretaceous (~ 100 Ma) brought global warming (Steuber et al, 2005), cold arctic winters followed for 30 million years (Davies et al, 2009). Fig.…”