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1. Ancient boreal forests under the environmental instability of the glacial to postglacial transition in the Great Lakes region (14 000 – 11 000 years BP)

   https://doi.org/10.1139/cjfr-2012-0339  

   Panyushkina, Irina P.; Leavitt, Steven W. (November 2013, Canadian Journal of Forest Research)

   Retreat of the Laurentide Ice Sheet 20 000 years ago tremendously altered environmental conditions and opened territory to the boreal spruce forest expansion. However, the details of forest colonization during the rapid climate warming and the adaptation of the newly developed stands to short cooling episodes during the warming and degradation of the ice sheet are not known. Preservation of wood from the glacial to postglacial transition offers the opportunity for examination of high-frequency growth variability in response to hemispheric and local forcings on temperature and hydrology. Here we consider growth of spruce at three sites from the interior of Northern America developed at ca. 13 700, 12 100, and 11 300 calibrated years before present (cal years BP), with well-replicated tree-ring chronologies spanning from 116 to 310 years. The data show at least two generations of trees established at each of the sites promoted by short, warm intervals. The tree mortality was variously affected by both cold conditions and the influence of rising water table and sediment burial. The history of these stands indicates breaks in forest colonization following a century (or two) of successful migrations. Interestingly, the thinning of the spruce forest did not seem to open pioneering opportunities for other tree species at those times. 

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2. Nonlinear Growth and Physiological Responses of White Spruce at North American Arctic Treeline

   https://doi.org/10.1029/2022JG007096  

   Lévesque, Mathieu; Andreu‐Hayles, Laia; D’Arrigo, Rosanne; Oelkers, Rose; Buckley, Brendan M (April 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Much is still unknown about the growth and physiological responses of trees to global change at the northern treeline. We combined tree‐ring width data with century‐long stable carbon and oxygen isotope records to investigate growth and physiological responses of white spruce at two treeline sites in the Canadian Arctic to concurrent increases in temperature, atmospheric CO₂concentration (c_a), and decline in sea ice extent over the past century. The tree‐ring records were assessed during three periods with contrasting climatic conditions: (a) the early 20th century warming, (b) the 1940–1970 cooling period, and (c) the anthropogenic late 20th century warming period. We found opposing growth trends between the two sites, but similar carbon isotope discrimination (Δ¹³C) and intrinsic water‐use efficiency (_iWUE) trajectories. While tree growth (defined as basal area increment) increased at the site nearer to the Arctic Ocean during the 20th century following the rise in temperature and sea ice loss, growth declined after 1950 at the more interior site. At both sites, Δ¹³C slightly increased over these periods. However, trees showed a nonlinear response to increasedc_a, shifting after 1970 from a passive stomatal response (i.e., no changes in_iWUE) to an active response (i.e., a moderate ∼12% increase in_iWUE). Further, our isotope‐based findings do not support the idea that temperature‐induced drought stress caused the divergent growth trends at our treeline sites. This study thus highlights nonlinear and complex physiological and growth adjustments to concomitant changes in temperature, sea ice extent, andc_aover the last century at the northern treeline. 

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3. Under‐Ice Phytoplankton Blooms Inhibited by Spring Convective Mixing in Refreezing Leads

   https://doi.org/10.1002/2016JC012575  

   Lowry, Kate E.; Pickart, Robert S.; Selz, Virginia; Mills, Matthew M.; Pacini, Astrid; Lewis, Kate M.; Joy‐Warren, Hannah L.; Nobre, Carolina; van Dijken, Gert L.; Grondin, Pierre‐Luc; et al (January 2018, Journal of Geophysical Research: Oceans)

   Abstract Spring phytoplankton growth in polar marine ecosystems is limited by light availability beneath ice‐covered waters, particularly early in the season prior to snowmelt and melt pond formation. Leads of open water increase light transmission to the ice‐covered ocean and are sites of air‐sea exchange. We explore the role of leads in controlling phytoplankton bloom dynamics within the sea ice zone of the Arctic Ocean. Data are presented from spring measurements in the Chukchi Sea during the Study of Under‐ice Blooms In the Chukchi Ecosystem (SUBICE) program in May and June 2014. We observed that fully consolidated sea ice supported modest under‐ice blooms, while waters beneath sea ice with leads had significantly lower phytoplankton biomass, despite high nutrient availability. Through an analysis of hydrographic and biological properties, we attribute this counterintuitive finding to springtime convective mixing in refreezing leads of open water. Our results demonstrate that waters beneath loosely consolidated sea ice (84–95% ice concentration) had weak stratification and were frequently mixed below the critical depth (the depth at which depth‐integrated production balances depth‐integrated respiration). These findings are supported by theoretical model calculations of under‐ice light, primary production, and critical depth at varied lead fractions. The model demonstrates that under‐ice blooms can form even beneath snow‐covered sea ice in the absence of mixing but not in more deeply mixed waters beneath sea ice with refreezing leads. Future estimates of primary production should account for these phytoplankton dynamics in ice‐covered waters. 

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4. A comprehensive satellite-based assessment across the Pacific Arctic Distributed Biological Observatory shows widespread late-season sea surface warming and sea ice declines with significant influences on primary productivity

   https://doi.org/10.1371/journal.pone.0287960  

   Frey, Karen E.; Comiso, Josefino C.; Stock, Larry V.; Young, Luisa N.; Cooper, Lee W.; Grebmeier, Jacqueline M. (July 2023, PLOS ONE)

   Westergaard-Nielsen, Andreas (Ed.)

   Massive declines in sea ice cover and widespread warming seawaters across the Pacific Arctic region over the past several decades have resulted in profound shifts in marine ecosystems that have cascaded throughout all trophic levels. The Distributed Biological Observatory (DBO) provides sampling infrastructure for a latitudinal gradient of biological “hotspot” regions across the Pacific Arctic region, with eight sites spanning the northern Bering, Chukchi, and Beaufort Seas. The purpose of this study is two-fold: (a) to provide an assessment of satellite-based environmental variables for the eight DBO sites (including sea surface temperature (SST), sea ice concentration, annual sea ice persistence and the timing of sea ice breakup/formation, chlorophyll- a concentrations, primary productivity, and photosynthetically available radiation (PAR)) as well as their trends across the 2003–2020 time period; and (b) to assess the importance of sea ice presence/open water for influencing primary productivity across the region and for the eight DBO sites in particular. While we observe significant trends in SST, sea ice, and chlorophyll- a /primary productivity throughout the year, the most significant and synoptic trends for the DBO sites have been those during late summer and autumn (warming SST during October/November, later shifts in the timing of sea ice formation, and increases in chlorophyll- a /primary productivity during August/September). Those DBO sites where significant increases in annual primary productivity over the 2003–2020 time period have been observed include DBO1 in the Bering Sea (37.7 g C/m 2 /year/decade), DBO3 in the Chukchi Sea (48.0 g C/m 2 /year/decade), and DBO8 in the Beaufort Sea (38.8 g C/m 2 /year/decade). The length of the open water season explains the variance of annual primary productivity most strongly for sites DBO3 (74%), DBO4 in the Chukchi Sea (79%), and DBO6 in the Beaufort Sea (78%), with DBO3 influenced most strongly with each day of additional increased open water (3.8 g C/m 2 /year per day). These synoptic satellite-based observations across the suite of DBO sites will provide the legacy groundwork necessary to track additional and inevitable future physical and biological change across the region in response to ongoing climate warming. 

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5. Early and middle Miocene ice sheet dynamics in the Ross Sea: Results from integrated core-log-seismic interpretation

   https://doi.org/10.1130/B35814.1  

   Pérez, Lara F.; Santis, Laura De; McKay, Robert M.; Larter, Robert D.; Ash, Jeanine; Bart, Phil J.; Böhm, Gualtiero; Brancatelli, Giuseppe; Browne, Imogen; Colleoni, Florence; et al (May 2021, GSA Bulletin)

   Abstract Oscillations in ice sheet extent during early and middle Miocene are intermittently preserved in the sedimentary record from the Antarctic continental shelf, with widespread erosion occurring during major ice sheet advances, and open marine deposition during times of ice sheet retreat. Data from seismic reflection surveys and drill sites from Deep Sea Drilling Project Leg 28 and International Ocean Discovery Program Expedition 374, located across the present-day middle continental shelf of the central Ross Sea (Antarctica), indicate the presence of expanded early to middle Miocene sedimentary sections. These include the Miocene climate optimum (MCO ca. 17–14.6 Ma) and the middle Miocene climate transition (MMCT ca. 14.6–13.9 Ma). Here, we correlate drill core records, wireline logs and reflection seismic data to elucidate the depositional architecture of the continental shelf and reconstruct the evolution and variability of dynamic ice sheets in the Ross Sea during the Miocene. Drill-site data are used to constrain seismic isopach maps that document the evolution of different ice sheets and ice caps which influenced sedimentary processes in the Ross Sea through the early to middle Miocene. In the early Miocene, periods of localized advance of the ice margin are revealed by the formation of thick sediment wedges prograding into the basins. At this time, morainal bank complexes are distinguished along the basin margins suggesting sediment supply derived from marine-terminating glaciers. During the MCO, biosiliceous-bearing sediments are regionally mapped within the depocenters of the major sedimentary basin across the Ross Sea, indicative of widespread open marine deposition with reduced glacimarine influence. At the MMCT, a distinct erosive surface is interpreted as representing large-scale marine-based ice sheet advance over most of the Ross Sea paleo-continental shelf. The regional mapping of the seismic stratigraphic architecture and its correlation to drilling data indicate a regional transition through the Miocene from growth of ice caps and inland ice sheets with marine-terminating margins, to widespread marine-based ice sheets extending across the outer continental shelf in the Ross Sea. 

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