Holocene climate changes in eastern Beringia (NW North America) – A systematic review of multi-proxy evidence

Abstract

Reconstructing climates of the past relies on a variety of evidence from a large number of sites to capture the varied features of climate and the spatial heterogeneity of climate 32 change. This review summarizes available information from diverse Holocene paleoenvironmental records across eastern Beringia (Alaska, westernmost Canada and adjacent 34 seas), and it quantifies the primary trends of temperature-and moisture-sensitive records based in part on midges, pollen, and biogeochemical indicators (compiled in the recently published Arctic 36Holocene database, and updated here to v2.1). The composite time series from these proxy records are compared with new summaries of mountain-glacier and lake-level fluctuations, 38 terrestrial water-isotope records, sea-ice and sea-surface-temperature analyses, and peatland and thaw-lake initiation frequencies to clarify multi-centennial-to millennial-scale trends in 40Holocene climate change. To focus the synthesis, the paleo data are used to frame specific questions that can be addressed with simulations by Earth system models to investigate the 42 causes and dynamics of past and future climate change. This systematic review shows that, during the early Holocene (11.7-8.2 ka), rather than a prominent thermal maximum as suggested 44 previously, temperatures were highly variable, at times both higher and lower than present (approximate mid-20 th -century average), with no clear spatial pattern. Composited pollen, midge 46 and other proxy records average out the variability and show the overall lowest summer and mean-annual temperatures across the study region during the earliest Holocene, followed by 48 warming over the early Holocene. The sparse data available on early Holocene glaciation show that glaciers in southern Alaska were as extensive then as they were during the late Holocene. 50Early Holocene lake levels were low in interior Alaska, but moisture indicators show pronounced differences across the region. The highest frequency of both peatland and thaw-lake initiation 52 ages also occurred during the early Holocene. During the middle Holocene (8.2-4.2 ka), glaciers 3 retreated as the regional average temperature increased to a maximum between 7 and 5 ka, as 54 reflected in most proxy types. Following the middle Holocene thermal maximum, temperatures decreased starting between 4 and 3 ka, signaling the onset of Neoglacial cooling. Glaciers in the 56 Brooks and Alaska Ranges advanced to their maximum Holocene extent as lakes generally rose to modern levels. Temperature differences for averaged 500-year time steps typically ranged by 58 1-2°C for individual records in the Arctic Holocene database, with a transition to a cooler late Holocene that was neither abrupt nor spatially coherent. The longest and highest-resolution 60 terrestrial water isotope records previously interpreted to represent changes in the Aleutian lowpressure system around this time are here shown to be largely contradictory. Furthermore, there 62 are too few records with sufficient resolution to ...

Central Arctic Ocean paleoceanography from ∼ 50 ka to present, on the basis of ostracode faunal assemblages from the SWERUS 2014 expedition

“…This increase in abundance is correlated with an increase in perennial sea ice, and is more prominent in cores from the Lomonosov Ridge than in cores from the Mendeleev Ridge (most likely due to more persistent perennial sea-ice cover over the Lomonosov Ridge sites). The inferred middle to late Holocene development of perennial sea ice is consistent with interpretations from other seaice proxies (Xiao et al, 2015) and with the transition from an early-middle Holocene "thermal maximum" (Kaufman et al, , 2016 to cooler conditions during the last few thousand years. (2003) and Poirier et al (2012), and core AOS94 28 (PI-94-AR-BC28, 1990 m) in Darby et al (1997).…”

Holocene dynamics in the Bering Strait inflow to the Arctic and the Beaufort Gyre circulation based on sedimentary records from the Chukchi Sea

“…4b). For example, multi-proxy data from the interior of Alaska and adjacent territories (Kaufman et al, 2016, and references therein) indicate overall drier and warmer conditions in the middle Holocene, consistent with a weaker Aleutian Low and stronger BSI. Diatom records from the southern Bering Sea indicate more abundant sea ice in the middle Holocene, also suggestive of a weaker Aleutian Low (Katsuki et al, 2009).…”

The last deglaciation of Alaska

“…This flooding event may have led to regional forcing, such as an increase in precipitation due to more northerly storm tracks (Kaufman et al, 2010) that may have influenced glacier mass balance. Of course, there could have been more glacier fluctuations during the Younger Dryas than are currently recognized because they may have occurred during a climate state that was warmer than the late Holocene (e.g., Kurek et al, 2009;Kaufman et al, 2016), and hence moraines were later obscured by subsequent glacier advances during the Holocene.…”

“…We build from the most recent review of the Pleistocene glacier history of Alaska (Kaufman et al, 2011). There have been some new glacial chronology studies published since that time, and furthermore, unlike past reviews spanning the Late Pleistocene (Briner and Kaufman, 2008;Kaufman et al, 2011) and spanning the Holocene (Barclay et al, 2009;Kaufman et al, 2016), this paper focuses solely on the last deglaciation. This is the first review paper on the glaciation history of Alaska to do so.…”