Gulfstream Variability in a Globally Warming World – The Forgotten Danish Archive

Geological archives from past warm periods are essential for contextualising future climate change under ongoing global warming. However, interpreting these archives requires a robust understanding of how palaeotemperature proxies record oceanographic variability under modern boundary conditions. This study presents ongoing Holocene alkenone-based sea surface temperature (SST) reconstructions from marine sediment cores collected around Denmark. The primary aim is to constrain alkenone signal provenance through comparison with instrumental SST datasets, and to apply this understanding to reconstructions of Gulfstream variability during past warm climates.

The Danish coastal seas, including the Skagerrak–Kattegat region and shelf settings along the Jutland Peninsula, occupy a climatically sensitive position at the interface between warm Atlantic waters transported by the North Atlantic Current and waters derived from the Nordic Seas, Baltic outflow, and terrestrial runoff. Variability in this Atlantic inflow has been linked to changes in North Atlantic heat transport and proposed as a sensitive indicator of broader AMOC-related variability. As such, the Danish marine realm offers a strategic location for assessing how surface ocean temperatures respond to circulation changes under differing climate states.

We use Holocene-age sediment cores to reconstruct SSTs using alkenone palaeothermometry, a biomarker-based proxy derived from marine haptophyte algae that records upper-ocean temperature conditions. Comparison of Holocene alkenone-derived SSTs with instrumental datasets provides a framework for assessing how proxy temperatures relate to observed surface ocean variability, including the influence of regional circulation changes, stratification, and potential freshwater input. This is of particular importance when examining alkenones, as these compounds may be transported by ocean currents, potentially biasing the recorded temperature signal if such effects are not accounted for. In addition to constraining proxy behaviour, the Holocene record is used to explore the expression of Holocene climate variability in the Danish coastal seas and its relationship to other North Atlantic records.

This Holocene–instrumental framework directly supports ongoing research investigating Gulfstream variability during the Eemian Interglacial (MIS 5e, or the Last Interglacial), a past warm period when the average global temperature was approximately 1–1.5 °C higher than present. Exceptionally thick Eemian marine clay sequences from the Vendsyssel region of Denmark are currently being analysed to develop high-resolution alkenone-based SST records capable of resolving multidecadal variability under warm-climate boundary conditions. Anchoring these reconstructions in a modern, provenance-sensitive analogue improves confidence in interpretations of Gulfstream behaviour during past warm periods and enhances the use of geological archives to inform expectations of future oceanographic change.