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1. Minimal stratigraphic evidence for coseismic coastal subsidence during 2000 yr of megathrust earthquakes at the central Cascadia subduction zone

   https://doi.org/10.1130/GES02254.1  

   Nelson, Alan R.; Hawkes, Andrea D.; Sawai, Yuki; Horton, Ben P.; Witter, Rob C.; Bradley, Lee-Ann; Cahill, Niamh (December 2020, Geosphere)

   Abstract Lithology and microfossil biostratigraphy beneath the marshes of a central Oregon estuary limit geophysical models of Cascadia megathrust rupture during successive earthquakes by ruling out >0.5 m of coseismic coastal subsidence for the past 2000 yr. Although the stratigraphy in cores and outcrops includes as many as 12 peat-mud contacts, like those commonly inferred to record subsidence during megathrust earthquakes, mapping, qualitative diatom analysis, foraminiferal transfer function analysis, and 14C dating of the contacts failed to confirm that any contacts formed through subsidence during great earthquakes. Based on the youngest peat-mud contact’s distinctness, >400 m distribution, ∼0.6 m depth, and overlying probable tsunami deposit, we attribute it to the great 1700 CE Cascadia earthquake and(or) its accompanying tsunami. Minimal changes in diatom assemblages from below the contact to above its probable tsunami deposit suggest that the lower of several foraminiferal transfer function reconstructions of coseismic subsidence across the contact (0.1–0.5 m) is most accurate. The more limited stratigraphic extent and minimal changes in lithology, foraminifera, and(or) diatom assemblages across the other 11 peat-mud contacts are insufficient to distinguish them from contacts formed through small, gradual, or localized changes in tide levels during river floods, storm surges, and gradual sea-level rise. Although no data preclude any contacts from being synchronous with a megathrust earthquake, the evidence is equally consistent with all contacts recording relative sea-level changes below the ∼0.5 m detection threshold for distinguishing coseismic from nonseismic changes. 

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2. Kill dates from re-exposed black mosses constrain past glacier advances in the northern Antarctic Peninsula

   Groff, V. (January 2023, Geology)

   Glaciers are receding in the northern Antarctic Peninsula and exposing previously entombed soils and plants. We used 39 black (dead) mosses collected from rapidly retreating ice margins at four sites along the Antarctic Peninsula to determine the kill dates using radiocarbon measurements and to constrain the timing of past glacier advances over the last 1500 yr. We established strict new criteria for sample collection to promote robust estimates of plant death. We found distinct phases of ice advance during ca. 1300, 800, and 200 calibrated years before 1950 (cal yr B.P.). We report estimates of the rate of glacier advance at ca. 800 cal yr B.P. at Gamage and Bonaparte Points (southern Anvers Island) of 2.0 and 0.3 m/yr, respectively. Although the range of kill dates is relatively narrow within a region, suggesting multiple glaciers advanced simultaneously, the rates of local advances can vary by almost an order of magnitude and are much less than retreat rates. Our kill dates coincide with evidence for glacier advances from other studies in the northern Antarctic Peninsula at ca. 1300, 800, and 200 cal yr B.P. and for penguin colony abandonment at several sites in the region ranging from 450 to 0 cal yr B.P. The combination of our new terrestrial evidence for glacier advances with other lines of evidence shows the regional synchroneity of glacial dynamics and cryosphere-biosphere connections during rapid climate shifts and the sensitivity of terrestrial ecosystems to climate cooling. 

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3. Late Holocene ice-mass changes recorded in a relative sea-level record from Joinville Island, Antarctica

   https://doi.org/10.1130/G46649.1  

   Zurbuchen, Julie; Simms, Alexander R. (September 2019, Geology)

   Abstract Recent ice-mass loss driven by warming along the Antarctic Peninsula has resulted in rapid changes in uplift rates across the region. Are such events only a function of recent warming? If not, does the Earth response to such events last long enough to be preserved in Holocene records of relative sea level (RSL), and thus have a bearing on global-scale glacial isostatic adjustment (GIA) models (e.g. ICE-6G)? Answering such questions in Antarctica is hindered by the scarcity of RSL reconstructions within the region. Here, a new RSL reconstruction for Antarctica is presented based on beach ridges from Joinville Island on the Antarctic Peninsula. We find that RSL has fallen 4.9 ± 0.58 m over the past 3100 yr, and that the island experienced a significant increase in the rate of RSL fall from 1540 ± 125 cal. (calibrated) yr B.P. to 1320 ± 125 cal. yr B.P. This increase in the rate of RSL fall is likely due to the viscoelastic response of the solid Earth to terrestrial ice-mass loss from the Antarctic Peninsula, similar to the Earth response experienced after ice-mass loss following acceleration of glaciers behind the collapsed Larsen B ice shelf in 2002 C.E. Additionally, slower rates of beach-ridge progradation from 695 ± 190 cal. yr B.P. to 235 ± 175 cal. yr B.P. potentially reflect erosion of beach ridges from a RSL rise induced by a local glacial advance. The rapid response of the Earth to minor ice-mass changes recorded in the RSL record further supports recent assertions of a more responsive Earth to glacial unloading and at time scales relevant for GIA of Holocene and Pleistocene sea levels. Thus, current continental and global GIA models may not accurately capture the ice-mass changes of the Antarctic ice sheets at decadal and centennial time scales. 

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4. Late Pleistocene and early Holocene sea-level history and glacial retreat interpreted from shell-bearing marine deposits of southeastern Alaska, USA

   https://doi.org/10.1130/GES02359.1  

   Baichtal, James F.; Lesnek, Alia J.; Carlson, Risa J.; Schmuck, Nicholas S.; Smith, Jane L.; Landwehr, Dennis J.; Briner, Jason P. (November 2021, Geosphere)

   Abstract We leverage a data set of >720 shell-bearing marine deposits throughout southeastern Alaska (USA) to develop updated relative sea-level curves that span the past ∼14,000 yr. This data set includes site location, elevation, description when available, and 436 14C ages, 45 of which are published here for the first time. Our sea-level curves suggest a peripheral forebulge developed west of the retreating Cordilleran Ice Sheet (CIS) margin between ca. 17,000 and 10,800 calibrated yr B.P. By 14,870 ± 630 to 12,820 ± 340 cal. yr B.P., CIS margins had retreated from all of southeastern Alaska's fjords, channels, and passages. At this time, isolated or stranded ice caps existed on the islands, with alpine or tidewater glaciers in many valleys. Paleoshorelines up to 25 m above sea level mark the maximum elevation of transgression in the southern portion of the study region, which was achieved by 11,000 ± 390 to 10,500 ± 420 cal. yr B.P. The presence of Pacific sardine (Sardinops sagax) and the abundance of charcoal in sediments that date between 11,000 ± 390 and 7630 ± 90 cal. yr B.P. suggest that both ocean and air temperatures in southeastern Alaska were relatively warm in the early Holocene. The sea-level and paleoenvironmental reconstruction presented here can inform future investigations into the glacial, volcanic, and archaeological history of southeastern Alaska. 

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5. Ancient Adélie penguin colony revealed by snowmelt at Cape Irizar, Ross Sea, Antarctica

   https://doi.org/10.1130/G48230.1  

   Emslie, Steven D. (September 2020, Geology)

   Abstract The Ross Sea (Antarctica) is one of the most productive marine ecosystems in the Southern Ocean and supports nearly one million breeding pairs of Adélie penguins (Pygoscelis adeliae) annually. There also is a well-preserved record of abandoned penguin colonies that date from before the Last Glacial Maximum (>45,000 14C yr B.P.) through the Holocene. Cape Irizar is a rocky cape located just south of the Drygalski Ice Tongue on the Scott Coast. In January 2016, several abandoned Adélie penguin sites and abundant surface remains of penguin bones, feathers, and carcasses that appeared to be fresh were being exposed by melting snow and were sampled for radiocarbon analysis. The results indicate the “fresh” remains are actually ancient and that three periods of occupation by Adélie penguins are represented beginning ca. 5000 calibrated calendar (cal.) yr B.P., with the last occupation ending by ca. 800 cal. yr B.P. The presence of fresh-appearing remains on the surface that are actually ancient in age suggests that only recently has snowmelt exposed previously frozen carcasses and other remains for the first time in ∼800 yr, allowing them to decay and appear fresh. Recent warming trends and historical satellite imagery (Landsat) showing decreasing snow cover on the cape since 2013 support this hypothesis. Increased δ13C values of penguin bone collagen further indicate a period of enhanced marine productivity during the penguin “optimum”, a warm period at 4000–2000 cal. yr B.P., perhaps related to an expansion of the Terra Nova Bay polynya with calving events of the Drygalski Ice Tongue. 

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