Towards a record of millennial-scale climate variability in the North Atlantic covering the past 1 million years, a biomarker approach (IODP Expedition 303/306)
Final Report Abstract
The results from this project highlight the dynamic climate of the North Atlantic during the Quaternary period. During the Pliocene, warm sea surface temperatures (SSTs) characterized the mid-latitude North Atlantic. The general absence of ice-rafted debris (rock particles transported to the open ocean by icebergs) in the mid-latitude North Atlantic suggests that large continental ice sheets were absent in the Northern Hemisphere. However, even this warm climate was characterized by at least one large glacial period (MIS M2, ~ 3.3 Ma) with ice sheets in the Northern Hemisphere that were larger than present. It is likely that a change in ocean circulation and reduction in northward heat transport was the ultimate trigger for MIS M2. As climate cooled during the latest Pliocene and early Pleistocene, SSTs in the North Atlantic cooled and icebergs started to regularly influence the region during glacials. Interglacials were still several degrees warmer than present and sea surface cooling was predominantly restricted to glacials. In the mid-latitude North Atlantic glacial periods were characterized by higher surface water productivity compared to interglacials. The opposite behavior is seen in the subpolar North Atlantic. This antiphase behavior is likely due to the southward movement of a narrow highproductivity band, forced by the expansion of glacial conditions in the high northern North Atlantic. Detailed biomarker data indicate that these shifts in productivity influenced all major groups of primary producers. Superimposed on these glacial/interglacial variations, the harsh conditions during ice-rafting events led to brief collapses of surface productivity. In addition, this project provided detailed information about the type of rock particles transported by icebergs to the North Atlantic and hence growth and decay of the circum-Atlantic continental ice sheets. During the early Pleistocene (2.5-1 Ma) the type of rock particles points towards a dominant source from Greenland and Scandinavia, suggesting that these continental ice sheets reached sea level during the early Pleistocene. As glacials intensified and the pacing of glacial/interglacial cycles shifted from 41-ka to 100-ka, rock particles from the Hudson Area in Northern Canada reached the mid-latitude North Atlantic (last 650 ka). These massive icerafting events, named Hudson Strait Heinrich Events, mark the episodic collapse of the Laurentide ice sheet that covered large parts of North America during glacials. These results lend further support for the existing hypothesis that an increase in size (thickness) of the Laurentide ice sheet controls the occurrence of North Atlantic HS Heinrich Events, favoring an internal dynamic mechanism for their occurrence. In general Heinrich Events are associated with the coolest SSTs in the midlatitude North Atlantic, reflecting the influence of melting icebergs and meltwater flux. However during the last 70 ka did SSTs increase during Heinrich Events. This was likely due to a unique oceanic configuration that led to a northward expansion of the subtropical gyre during these specific Heinrich events. The transition from the last glacial into the Holocene was interrupted by the Younger Dryas cooling event during which large parts of the Northern Hemisphere returned to near-glacial conditions for a brief period. Results from this project indicate that the YD was likely related to a collapse of the Laurentide ice sheet, as regularly occurred during Hudson Strait Heinrich Events of the last 650 ka, that caused a reduction in overturning circulation and does not request a unique trigger.
Publications
- 2012. A Laurentide outburst flooding event during the last interglacial period. Nature Geoscience, Vol. 5.2012, Issue 12, pp. 901-904.
Nicholl, J.A.L., Hodell, D.A., Naafs, B.D.A., Hillaire-Marcel, C., Channell, J.E.T., Romero, O.E.
(See online at https://doi.org/10.1038/NGEO1622) - 2013. Millennial-scale ice rafting events and Hudson Strait Heinrich(-like) Events during the late Pliocene and Pleistocene: a review. Quaternary Science Reviews, Vol. 80. 2013, pp. 1–28.
Naafs, B.D.A., Hefter, J., Stein, R.
(See online at https://doi.org/10.1016/j.quascirev.2013.08.014)
