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Warming in the North Atlantic Ocean has been heterogeneous in recent decades, with locations along the eastern United States seaboard (northwestern Atlantic) seeing some of the largest and fastest warming in the last 100 years. In order to provide a longer temporal context for these changes, we are in the process of developing several master shell growth chronologies and associated geochemical records from theMid-Atlantic coast using the shells of the long-lived marine bivalve Arctica islandica. Based on the shell collection locations (shelf regions offOcean City, Maryland in ~ 61 m water depth and Long Island, New York in ~47 m water depth) and shell geochemistry measurements, we will be able to better ascertain hydrographic spatial and temporal variability of subtropical Atlantic water moving northward through time. These findings will be integrated with similar sclerochronology datasets previously published from the Gulf of Maine region and several others from theMid-Atlantic region that are currently being constructed. Collectively, this network of sclerochronology records will allow us to better characterize changes in the northwestern Atlantic and provide hydrographic insights beyond the relatively short instrumental record and evaluate potential dynamical forcings through time. 

The Indian Summer Monsoon [ISM] provides approximately 80% of South Asia’s annual average precipitation. Nepal represents a particularly important sector of the ISM because of its location at the base of the Himalayas, Asia’s water tower, and in the zone of influence of the mid-latitude westerlies. Late Holocene ISM variability has previously been examined using high resolution resolved stable isotope records of stalagmites from northern, northeastern, and central India, but as of yet, no such records have been published from Nepal. We present high resolution stable isotopic time series from two precisely-dated and partially overlapping stalagmites spanning the last 2400 years from Siddha Baba Cave, central Nepal, as well as a year of isotopic data from rainwater collected near the cave. It has been suggested that the amount effect has only a minor effect on the oxygen isotope variability in precipitation in this area. As a result, we couple oxygen and carbon isotopes from these stalagmites to examine both regional and local-scale ISM dynamics. The Siddha Baba record reveals two periods suggestive of changes in the ISM: an apparent increase in rainfall during approximately CE 1350-1550 and a reduction in rainfall characterizing the last two centuries. We investigate these intervals using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model. A primary focus is on links between Indo-Pacific ocean-atmosphere interactions and subsequent changes in the monsoon circulation over the Indian subcontinent, as well as regional moisture transport into Nepal between these periods. 

The Indian Summer Monsoon [ISM] provides approximately 80% of South Asia’s annual average precipitation. Nepal represents a particularly important sector of the ISM because of its location at the base of the Himalayas, Asia’s water tower, and in the zone of influence of the mid-latitude westerlies. Late Holocene ISM variability has previously been examined using high resolution resolved stable isotope records of stalagmites from northern, northeastern, and central India, but as of yet, no such records have been published from Nepal. We present high resolution stable isotopic time series from two precisely-dated and partially overlapping stalagmites spanning the last 2400 years from Siddha Baba Cave, central Nepal, as well as a year of isotopic data from rainwater collected near the cave. It has been suggested that the amount effect has only a minor effect on the oxygen isotope variability in precipitation in this area. As a result, we couple oxygen and carbon isotopes from these stalagmites to examine both regional and local-scale ISM dynamics. The Siddha Baba record reveals two periods suggestive of changes in the ISM: an apparent increase in rainfall during approximately CE 1350-1550 and a reduction in rainfall characterizing the last two centuries. We investigate these intervals using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model. A primary focus is on links between Indo-Pacific ocean-atmosphere interactions and subsequent changes in the monsoon circulation over the Indian subcontinent, as well as regional moisture transport into Nepal between these periods. 

Abstract The Gulf of Maine and surrounding western North Atlantic shelf are some of the fastest warming regions of the worlds oceans. The lack of long-term observational records from this area inhibits the ability to assess the timing and initial causes of this warming and consequently accurately predict future changes to this ecologically and economically important region. Here we present oxygen, nitrogen, and radiocarbon isotope data measured in Arctica islandica shells collected in the western North Atlantic to better understand the past temperature and ocean circulation variability of the region over the last 300 years. We combine these results with output from the Community Earth System Model Last Millennium Ensemble simulations to assess the temporal and spatial context of these isotope records. We find that the isotope records capture the end and reversal of a millennium-scale cooling trend in the Gulf of Maine. Last Millennium Ensemble single-forcing simulations indicate that this cooling trend appears to be largely driven by volcanic forcing. The nitrogen and radiocarbon records indicate that ocean circulation is in part driving the reconstructed hydrographic changes, pointing to a potential role of the Atlantic Meridional Overturning Circulation in regulating Gulf of Maine temperatures as suggested by the Last Millennium Ensemble simulations. Both isotope and model results suggest that the Gulf of Maine began to warm in the late 19th century, ultimately driven by increased greenhouse gas forcing. Plain-language Summary The Gulf of Maine, located off of the Eastern Coast of the United States, has experienced significant temperature increases recently. Because the instrumental record only began in 1905, we do not have a good idea of when this warming began and what may have initially caused the warming. Here, we analyze the chemistry of clam shells, which have grown in the Gulf of Maine for hundreds of years, to infer past changes in ocean temperatures and water properties. We combine these results with output from a climate model to reveal that the temperatures reconstructed from the clams shells agree well with the model during the period of overlap. Both the chemical records and the model suggest the Gulf of Maine started warming in the late 1800s as a result of increased atmospheric greenhouse gas concentrations. Before this warming began, the Gulf of Maine region appears to have been cooling. The model suggests that this cooling trend is likely due to the influence of volcanic eruptions. The chemical records from the clam shells also suggest that part of this cooling is likely related to changing ocean circulation patterns.