Extent and chronology of glaciations in Iceland; a brief overview of the glacial history

The onset of Quaternary glaciations is a critical event in the climate and tectonic history of the Tibetan Plateau. The Shishapangma Glaciation, defined based on the till deposit from the northern slopes of Mt. Shishapangma, has been identified as the oldest glaciation on the Tibetan Plateau. However, the timing of this glaciation has not been constrained. We measured ¹⁰Be and ²⁶Al concentrations of a set of boulders on top of this till and simulated their complex exposure-burial histories. The simulated results indicate that the formation age of this till is likely around 835.2 ± 241.0 ka, representing the minimum timing of glacial onset on the Tibetan Plateau. The Shishapangma Glaciation is apparently much younger than the glacial onset in many other areas of the world, such as Europe and North America, and was likely driven by the coupled effect between tectonic uplift and climate cooling during the early–middle Pleistocene transition.

The Pliocene climate is globally warm and characterised by high atmospheric carbon dioxide concentrations, yet the terrestrial and marine scientific communities have gathered considerable evidence for substantial glaciation events in both the Northern and Southern Hemisphere prior to the Quaternary. Evidence on land is fragmentary, but marine records of glaciation present a more complete history of Pliocene glaciation. Here we present a global compilation of the terrestrial and marine glacial evidence for the Pliocene and demonstrate four glaciation events that can be identified in the Southern and/or Northern Hemisphere prior to the latest Pliocene intensification of Northern Hemisphere glaciation. There are two globally recognisable glacial events in the early Pliocene (c. 4.9–4.8 Ma and c. 4.0 Ma), one event around the early/late Pliocene transition (c. 3.6 Ma), and one event during Marine Isotope Stage M2 (c. 3.3 Ma). Long-term climate cooling, decreasing carbon dioxide concentrations in the atmosphere and high climate sensitivity in the Pliocene probably facilitated each glaciation event, however the mechanisms behind the early Pliocene glacial events are unclear. The global glaciation at c. 3.3 Ma may be caused by changes in ocean gateways, whereas the decline in carbon dioxide concentrations is important for the latest Pliocene intensification of Northern Hemisphere glaciation.

Despite the undisputed role of the Arctic Ocean in the modern and Pliocene climate system, the Arctic has only recently attracted public awareness that ongoing, fundamental change in the Arctic cryosphere could be a response to global warming. Clarification of the Arcticʼs role in global climate during the Pliocene is, however, largely hampered by equivocal stratigraphic constraints. From a well-dated Pliocene sequence from the Yermak Plateau, off NW Spitsbergen, we present sedimentological and geochemical data indicating that 4 million years ago terrigenous sediment supply and sources changed abruptly in response to a regional tectonic uplift event. We argue that this event together with contemporary uplift and tilting along the northwestern European continental margin preconditioned the landmasses for glacial ice build-up during intensification of the Northern Hemisphere Glaciation (INHG). Our data further suggest that the final deepening/widening of the Arctic–Atlantic gateway, the Fram Strait, between 6.5 and 5 Ma gradually caused increased deep-water mass exchange which, in turn, likely contributed to the intensification of the North Atlantic thermohaline circulation. Coupled to the North Atlantic warm pool as a regional moisture source, declining atmospheric CO₂ levels and other feedback mechanisms during the Pliocene, the regional tectonic activities in the high northern latitudes caused decreased summer ablation and thus allowed the initial build-up of glacial ice both in Scandinavia, and the sub-aerially exposed Svalbard/Barents Sea, culminating in the first large-scale coastline-shelf edge glaciations at $\sim 2.75\text{Ma}$ ago.

The Plio-Pleistocene intensification of Northern Hemisphere continental ice-sheet development is known to have profoundly affected the global climate system. Evidence for early continental glaciation is preserved in sediments throughout the North Atlantic Ocean, where ice-rafted detritus (IRD) layers attest to the calving of sediment-loaded icebergs from circum-Atlantic ice sheets. So far, Early-Pleistocene IRD deposition has been attributed to the presence of high-latitudinal ice sheets, whereas the existence and extent of ice accumulation in more temperate, mid-latitudinal regions remains enigmatic.

Here we present results from the multiproxy provenance analysis of a unique, Pleistocene-Holocene IRD sequence from the Irish NE Atlantic continental margin. There, the Challenger coral carbonate mound (IODP Expedition 307 site U1317) preserved an Early-Pleistocene record of 16 distinctive IRD events, deposited between ca 2.6 and 1.7 Ma. Strong and complex IRD signals are also identified during the mid-Pleistocene climate transition (ca 1.2 to 0.65 Ma) and throughout the Middle-Late Pleistocene interval. Radiogenic isotope source-fingerprinting, in combination with coarse lithic component analysis, indicates a dominant sediment source in the nearby British–Irish Isles, even for the oldest, Early-Pleistocene IRD deposits. Hence, our findings demonstrate, for the first time, repeated and substantial (i.e. marine-terminating) ice accumulation on the British–Irish Isles since the beginning of the Pleistocene. Contemporaneous expansion of both high- and mid-latitudinal ice sheets in the North Atlantic region is therefore implied at the onset of the Pleistocene. Moreover, it suggests the recurrent establishment of (climatically) favourable conditions for ice sheet inception, growth and instability in mid-latitudinal regions, even in the earliest stages of Northern Hemisphere glacial expansion and in an obliquity-driven climate system.

The Quaternary is synonymous with extensive glaciation of Earth's mid- and high-latitudes. Although there were local precursors, significant glaciation began in the latest Eocene (ca 35 Ma) in eastern Antarctica. It was followed by glaciation in mountain areas through the Miocene (in Alaska, Greenland, Iceland and Patagonia), later in the Pliocene (e.g. in the Bolivian Andes and possibly in Tasmania) and in the earliest Pleistocene (e.g. in the Alps, New Zealand, Iceland and Greenland). Today, evidence from both the land and the ocean floors demonstrates that the major continental glaciations, outside the polar regions, rather than occurring throughout the 2.6 Ma of the Quaternary, were markedly restricted to the last 1 Ma—800 ka or less. Marine Isotope Stage (MIS) 22 (ca 870–880 ka) included the first of the ‘major’ worldwide events with substantial ice volumes that typify the Later Pleistocene glaciations (i.e. MIS 16, 12, 10, 6, 4-2).