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  1. Abstract

    The Faroe Islands, a North Atlantic archipelago between Norway and Iceland, were settled by Viking explorers in the mid-9th century CE. However, several indirect lines of evidence suggest earlier occupation of the Faroes by people from the British Isles. Here, we present sedimentary ancient DNA and molecular fecal biomarker evidence from a lake sediment core proximal to a prominent archaeological site in the Faroe Islands to establish the earliest date for the arrival of people in the watershed. Our results reveal an increase in fecal biomarker concentrations and the first appearance of sheep DNA at 500 CE (95% confidence interval 370-610 CE), pre-dating Norse settlements by 300 years. Sedimentary plant DNA indicates an increase in grasses and the disappearance of woody plants, likely due to livestock grazing. This provides unequivocal evidence for human arrival and livestock disturbance in the Faroe Islands centuries before Viking settlement in the 9th century.

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  2. Abstract

    High resolution seafloor mapping shows extraordinary evidence that massive (>300 m thick) icebergs once drifted >5,000 km south along the eastern United States, with >700 iceberg scours now identified south of Cape Hatteras. Here we report on sediment cores collected from several buried scours that show multiple plow marks align with Heinrich Event 3 (H3), ~31,000 years ago. Numerical glacial iceberg simulations indicate that the transport of icebergs to these sites occurs during massive, but short-lived, periods of elevated meltwater discharge. Transport of icebergs to the subtropics, away from deep water formation sites, may explain why H3 was associated with only a modest increase in ice-rafting across the subpolar North Atlantic, and implies a complex relationship between freshwater forcing and climate change. Stratigraphy from subbottom data across the scour marks shows there are additional features that are both older and younger, and may align with other periods of elevated meltwater discharge.

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  3. Abstract

    Growth‐increment widths of Pacific geoduck (Panopea generosa), a long‐lived bivalve, are used to develop the first marine‐based, multicentennial, annually resolved, and exactly dated archive of Northeast Pacific sea surface temperatures (SST). The chronology is sampled from the Tree Nob Islands, British Columbia, Canada, continuously covers 1725–2008, and also contains nine older radiocarbon‐dated segments, which together span 58% of the last 1,500 years. Age‐related growth declines were removed by aligning all increments relative to age of increment formation and fitting with a single detrending curve to preserve low‐frequency signals. The geoduck chronology was used to reconstruct local SST variability across the seasonal window of April through November. The chronology at both the concurrent (lag‐0) and following (lag+1) year are both highly significant predictors of SST in a stepwise multiple linear regression, explaining 54% of the variance in the period of instrumental overlap (1940–2001), passing strict tests of calibration‐verification. Reconstructed SSTs contained significant spectral power at periods from 3 to 64 years, suggesting that 20th century variability in these periodicities is not unusual in the longer‐term context. The period of lowest growth coincided with the Dalton minimum, an episode of reduced solar irradiance from 1790–1830, as well as the 1809 Unknown eruption, suggesting that solar and volcanic signals are present in the SST history. The most conspicuous aspect of the reconstruction is the steady and unprecedented warming trend that began in the mid‐1800s and continues through present. The post‐1976 interval includes the two warmest decades of the reconstruction.

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  4. Abstract

    The deep ocean has long been recognized as the reservoir that stores the carbon dioxide (CO2) removed from the atmosphere during Pleistocene glacial periods. The removal of glacial atmospheric CO2into the ocean is likely modulated by an increase in the degree of utilization of macronutrients at the sea surface and enhanced storage of respired CO2in the deep ocean, known as enhanced efficiency of the biological pump. Enhanced biological pump efficiency during glacial periods is most easily documented in the deep ocean using proxies for oxygen concentrations, which are directly linked to respiratory CO2levels. We document the enhanced storage of respired CO2during the Last Glacial Maximum (LGM) in the Pacific Southern Ocean and deepest Equatorial Pacific using records of deglacial authigenic manganese, which form as relict peaks during increases in bottom water oxygen (BWO) concentration. These peaks are found at depths and regions where other oxygenation histories have been ambiguous, due to diagenetic alteration of authigenic uranium, another proxy for BWO. Our results require that the entirety of the abyssal Pacific below approximately 1,000 m was enriched in respired CO2and depleted in oxygen during the LGM. The presence of authigenic Mn enrichment in the deep Equatorial Pacific for each of the last five deglaciations suggests that the storage of respired CO2in the deep ocean is a ubiquitous feature of late‐Pleistocene ice ages.

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  5. Abstract

    Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

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  6. Abstract

    The occurrence and magnitude of natural fossil methane (CH4) emissions in the Arctic are poorly known. Emission of geologic CH4, a potent greenhouse gas, originating beneath permafrost is of particular interest due to the potential for positive feedback to climate warming, whereby accelerated permafrost thaw releases permafrost‐trapped CH4in a future warmer climate. The development of through‐going taliks in Arctic lakes overlying hydrocarbon reservoirs is one mechanism of releasing geologically sourced, subpermafrost CH4. Here we use novel gas flux measurements, geophysical observations of the subsurface, shallow sediment coring, high‐resolution bathymetry measurements, and lake water chemistry measurements to produce a synoptic survey of the gas vent system in Esieh Lake, a northwest Alaska lake with exceedingly large geologic CH4seep emissions. We find that microbially produced fossil CH4is being vented though a narrow thaw conduit below Esieh Lake through pockmarks on the lake bottom. This is one of the highest flux geologic CH4seep fields known in the terrestrial environment and potentially the highest flux single methane seep. The poleward retreat of continuous permafrost may have implications for more subcap CH4release with increased permafrost thaw.

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  7. Abstract

    Stordalen Mire is a peatland in the discontinuous permafrost zone in arctic Sweden that exhibits a habitat gradient from permafrost palsa, toSphagnumbog underlain by permafrost, toEriophorum‐dominated fully thawed fen. We used three independent approaches to evaluate the annual, multi‐decadal, and millennial apparent carbon accumulation rates (aCAR) across this gradient: seven years of direct semi‐continuous measurement of CO2and CH4exchange, and 21 core profiles for210Pb and14C peat dating. Year‐round chamber measurements indicated net carbon balance of −13 ± 8, −49 ± 15, and −91 ± 43 g C m−2 y−1for the years 2012–2018 in palsa, bog, and fen, respectively. Methane emission offset 2%, 7%, and 17% of the CO2uptake rate across this gradient. Recent aCAR indicates higher C accumulation rates in surface peats in the palsa and bog compared to current CO2fluxes, but these assessments are more similar in the fen. aCAR increased from low millennial‐scale levels (17–29 g C m−2 y−1) to moderate aCAR of the past century (72–81 g C m−2 y−1) to higher recent aCAR of 90–147 g C m−2 y−1. Recent permafrost collapse, greater inundation and vegetation response has made the landscape a stronger CO2sink, but this CO2sink is increasingly offset by rising CH4emissions, dominated by modern carbon as determined by14C. The higher CH4emissions result in higher net CO2‐equivalentemissions, indicating that radiative forcing of this mire and similar permafrost ecosystems will exert a warming influence on future climate.

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  8. Abstract

    Bedrock landsliding, including the formation of landslide dams, is a predominant geomorphic process in steep landscapes. Clarifying the importance of hydrologic and seismic mechanisms for triggering deep‐seated landslides remains an ongoing effort, and formulation of geomorphic metrics that predict dam preservation is crucial for quantifying secondary landslide hazards. Here, we identify >200 landslide‐dammed lakes in western Oregon and utilize dendrochronology and enhanced14C dating (“wiggle matching”) of “ghost forests” to establish slope failure timing at 20 sites. Our dated landslide dataset reveals bedrock landsliding has been common since the last Cascadia Subduction Zone earthquake in January 1700 AD. Our study does not reveal landslides that date to 1700 AD. Rather, we observe temporal clustering ofat leastfour landslides in the winter of 1889/1890 AD, coincident with a series of atmospheric rivers that generated one of the largest regionally recorded floods. We use topographic and field analyses to assess the relation between dam preservation and topographic characteristics of the impounded valleys. In contrast to previous studies, we do not observe systematic scaling between dam size and upstream drainage area, though dam stability indices for our sites correspond with “stable” dams elsewhere. Notably, we observe that dams are preferentially preserved at drainage areas of ∼1.5 to 13 km2and valley widths of ∼25 to 80 m, which may reflect the reduced downstream influence of debris flows and the accumulation of mature conifer trees upstream from landslide‐dammed lake outlets. We suggest that wood accumulation upstream of landslide dams tempers large stream discharges, thus inhibiting dam incision.

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  9. Abstract

    Climate‐driven thawing of Arctic permafrost renders its vast carbon reserves susceptible to microbial degradation, serving as a potentially potent positive feedback hidden within the climate system. While seemingly intuitive, the relationship between thermally driven permafrost losses and organic carbon (OC) export remains largely unexplored in natural settings. Filling this knowledge gap, we present down‐core bulk and compound‐specific radiocarbon records of permafrost change from a sediment core taken within the Alaskan Colville River delta spanning the lastc. 2,700 years. Fingerprinted by significantly older radiocarbon ages of bulk OC and long‐chain fatty acids, these data expose a thermally driven increase in permafrost OC export and/or deepening of mobilizable permafrost layers over the lastc. 160 years after the Little Ice Age. Comparison of OC content and radiocarbon data between recent and Roman warming episodes likely implies that the rate of warming, alongside the prevailing boundary conditions, may dictate the ultimate fate of the Arctic's permafrost inventory. Our findings highlight the importance of leveraging geological records as archives of Arctic permafrost mobilization dynamics with temperature change.

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  10. Abstract

    The extent to which terrestrial organic matter supports aquatic consumers remains uncertain because factors regulating resource flows are poorly understood. We sampled 12 lakes throughout the Sierra Nevada (California, USA) spanning large gradients in elevation and size to evaluate how watershed attributes and lake morphometry influence resource flows to lake carbon pools and zooplankton. We found that the size and composition of carbon pools in lakes were often more strongly determined by watershed or lake features rather than by elevational position. Using three different tracers of resource origin (δ13C, Δ14C, C:N ratio), we found terrestrial contributions to most lake resource pools (dissolved organic carbon, particulate organic matter (POM), sediments) and pelagic consumers (zooplankton) were more strongly related to local‐scale watershed features such as vegetation cover or watershed area: lake area rather than to elevation. Landscape patterns in multiple tracers indicated consistent contribution of within‐lake C sources to bulk resource pools across elevations (POM, sediments, zooplankton). δ13C‐enrichment of lake C pools and overlap with δ13C of terrestrial resources can arise due to reduced fractionation of13C by phytoplankton under CO2limitation, therefore we recommend careful consideration of potential environmental drivers when interpreting among‐lake patterns in δ13C. Our findings emphasize the importance of local‐scale variation in mediating terrestrial contributions to lake food webs.

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