Synchronous retreat of Thwaites and Pine Island glaciers in response to external forcings in the presatellite era.

Today, relatively warm Circumpolar Deep Water is melting Thwaites Glacier at the base of its ice shelf and at the grounding zone, contributing to significant ice retreat. Accelerating ice loss has been observed since the 1970s; however, it is unclear when this phase of significant melting initiated. We analyzed the marine sedimentary record to reconstruct Thwaites Glacier's history from the early Holocene to present. Marine geophysical surveys were carried out along the floating ice-shelf margin to identify core locations from various geomorphic settings. We use sedimentological data and physical properties to define sedimentary facies at seven core sites. Glaciomarine sediment deposits reveal that the grounded ice in the Amundsen Sea Embayment had already retreated to within ~45 km of the modern grounding zone prior to ca. 9,400 y ago. Sediments deposited within the past 100+ y record abrupt changes in environmental conditions. On seafloor highs, these shifts document ice-shelf thinning initiating at least as early as the 1940s. Sediments recovered from deep basins reflect a transition from ice proximal to slightly more distal conditions, suggesting ongoing grounding-zone retreat since the 1950s. The timing of ice-shelf unpinning from the seafloor for Thwaites Glacier coincides with similar records from neighboring Pine Island Glacier. Our work provides robust new evidence that glacier retreat in the Amundsen Sea was initiated in the mid-twentieth century, likely associated with climate variability.

A 7000-year record of floods and ecological feedbacks in Weeks Bay, Alabama, USA.

Climate change, sea-level rise, and human activities present major concerns for coastal environments. Paleoenvironmental records allow us to extend the instrumented record and study recent environmental impacts in a long-term context with natural pre-industrial conditions. Here, we investigate grain size, stable carbon (δ13C) and nitrogen (δ15N) isotopes, elemental composition, and diatom abundance in sediments to construct a 7000-year paleoenvironmental history of Weeks Bay, Alabama, a NOAA National Estuarine Research Reserve. Four major floods of the Fish River since 1986 CE are independently identified in the Weeks Bay sediment record, validating the bay setting as an archive of flood events. Thirty-four flood events were identified over the last 5000 years, with two periods of intense flood activity coinciding with the Medieval Climate Anomaly and the Little Ice Age, indicating association of relatively short-term climate events and enhanced storm activity. Further, multiple paleoenvironmental proxies indicate marine conditions during formation of the bay ~6600 calendar years Before Present (cal yr BP) and a brackish transition as the estuary became restricted ~2000 cal yr BP. High total organic carbon/nitrogen values indicate nitrogen limitation in Weeks Bay. Increase in organic content, diatoms, and redox- and nutrient-associated elemental proxies over the last 300 years, with dramatic increase in algal abundance since the 1980s, strongly suggest that human activities (i.e. land clearing, agriculture) increased ecological feedbacks in the bay. Comparing past and present environmental conditions of coastal estuaries advances our understanding of estuarine response to climate change and sea level, floods, and human activities, which is important for environmental management and wetland conservation policy.