Alkenones and coccoliths in ice‐rafted debris during the Last Glacial Maximum in the North Atlantic: implications for the use of U^K₃₇′ as a sea surface temperature proxy

Abstract

The U K 37 0 index has proven to be a robust proxy to estimate past sea surface temperatures (SSTs) over a range of time scales, but like any other proxy, it has uncertainties. For instance, in reconstructions of the Last Glacial Maximum (LGM) in the northern North Atlantic, U K 37 0 indicates higher temperatures than those derived from foraminiferal proxies. Here we evaluate whether such warm glacial estimates are caused by the advection of reworked alkenones in ice-rafted debris (IRD) to deep-sea sediments. We have quantified both coccolith assemblages and alkenones in sediments from glaciogenic debris flows in the continental margins of the northern North Atlantic, and from a deep-sea core from the Reykjanes Ridge. Certain debris flow deposits in the North Atlantic were generated by the presence of massive ice-sheets in the past, and their associated ice streams. Such deposits are composed of the same materials that were present in the IRD at the time they were generated. We conclude that ice rafting from some locations was a transport pathway to the deep sea floor of reworked alkenones and pre-Quaternary coccolith species during glacial stages, but that not all of the IRD contained alkenones, even when reworked coccoliths were present. We speculate that the ratio of reworked coccoliths to alkenone concentration might be useful to infer whether significant reworked alkenone inputs from IRD did occur at a particular site in the glacial North Atlantic. We also observe that alkenones in some of the debris flows contain a colder signal than estimated for LGM sediments in the northern North Atlantic. This is also clear in the deep-sea core studied where the warmest intervals do not correspond to the intervals with large inputs of reworked coccoliths or IRD. We conclude that any possible bias to U K 37 0 estimates associated with reworked alkenones is not necessarily towards higher values, and that the high SST anomalies for the LGM are unlikely to be the result of a bias caused by IRD inputs.

Biomarker Fingerprint of Debris Flow Deposits as a Paleoproxy for IRD Sources in the Last Glacial North Atlantic

“…It is nonetheless worth noting that chlorins were detected in GDF samples from the North Sea Fan and Svalbard depocenter while absent from any other GDF sediments analyzed here (see supporting information Figure S2). Interestingly, the presence of alkenones was also reported in GDF sediments from these two depocenters (Rosell‐Melé et al, 2011). The occurrence of these compounds in marine sediments is usually attributed to autochthonous inputs.…”

“…The reader is referred to the supporting information from Rosell‐Melé et al (2011; https://onlinelibrary.wiley.com/doi/abs/10.1002/jqs.1488) for further description of these trough‐mouth fans and the corresponding analyzed cores.…”

Warming of surface waters in the mid‐latitude North Atlantic during Heinrich events

“…In addition, Rosell‐Melé et al . [] demonstrated that the input of reworked alkenones by IRD in the North Atlantic does not necessarily lead to higher alkenone‐based SSTs in marine sediments. Finally, during older glacials, ice‐rafting events at Site U1313 are always accompanied by abrupt cooling episodes [ Stein et al ., ; Naafs et al ., ].…”

Response to Comments on ‘Does the Agulhas Current amplify global temperatures during super‐interglacials?’

“…(2011) show that during the last glacial there can be different temperature trends as represented by different marine proxies. This is certainly something to be mindful of and forms an extensive and current area of research during this period (Rosell‐Melé et al ., 2011). We selected all sites where the original authors considered the temperature estimates of annual temperature to be reliable.…”