Open ocean convection drives enhanced eastern pathway of the Glacial Atlantic Meridional Overturning Circulation.

Abundant proxy records suggest a profound reorganization of the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM, ~21,000 y ago), with the North Atlantic Deep Water (NADW) shoaling significantly relative to the present-day (PD) and forming Glacial North Atlantic Intermediate Water (GNAIW). However, almost all previous observational and modeling studies have focused on the zonal mean two-dimensional AMOC feature, while recent progress in the understanding of modern AMOC reveals a more complicated three-dimensional structure, with NADW penetrating from the subpolar North Atlantic to lower latitude through different pathways. Here, combining 231Pa/230Th reconstructions and model simulations, we uncover a significant change in the three-dimensional structure of the glacial AMOC. Specifically, the mid-latitude eastern pathway (EP), located east of the Mid-Atlantic Ridge and transporting about half of the PD NADW from the subpolar gyre to the subtropical gyre, experienced substantial intensification during the LGM. A greater portion of the GNAIW was transported in the eastern basin during the LGM compared to NADW at the PD, resulting in opposite 231Pa/230Th changes between eastern and western basins during the LGM. Furthermore, in contrast to the wind-steering mechanism of EP at PD, the intensified LGM EP was caused primarily by the rim current forced by the basin-scale open-ocean convection over the subpolar North Atlantic. Our results underscore the importance of accounting for three-dimensional oceanographic changes to achieve more accurate reconstructions of past AMOC.

A nationwide registry-based observational study of thyroid disease incidence in the Faroe Islands.

OBJECTIVE: The occurrence of thyroid disease varies among populations. While the iodine nutrition level of the Faroese seems to have been decreasing over the past decades, there is no systematic evaluation of the thyroid disease pattern in the Faroe Islands. Such knowledge of thyroid disease occurrence in the North Atlantic region may support healthcare planning and prevention. To investigate incidence rates, including subtypes of thyroid diseases, and demographic characteristics of thyroid disease patients in the Faroe Islands, to improve understanding of the patterns and trends of these disorders. DESIGN AND METHOD: A registry-based observational study was conducted over 10 years, encompassing all adult Faroese individuals. PATIENTS AND MEASUREMENTS: Health records from general practitioners and hospitals were used to identify incident cases of thyroid diseases. Validation was performed using multiple data sources. The incidence rates were standardised using population data from the middle of the study period 2006-2018. RESULTS: Among the 1152 individuals diagnosed with thyroid disease, the standardised incidence rates per 100,000 person-years were 55 for hyperthyroidism and 112 for hypothyroidism, and around four times higher in women than in men. Hashimoto's thyroiditis was the dominant cause of hypothyroidism, while Graves' disease was the leading cause of hyperthyroidism. The incidence of hypothyroidism increases with age. A decreasing trend was observed over time for both hypothyroidism and hyperthyroidism. CONCLUSION: Considering the decrease in iodine nutrition levels over the past decades, we were surprised by the high incidence of autoimmune thyroid disease. The findings highlight the need for continuous monitoring of thyroid disease occurrence in coastal areas of the North Atlantic Ocean.

Climatic effects on the synchrony and stability of temperate headwater invertebrates over four decades.

Important clues about the ecological effects of climate change can arise from understanding the influence of other Earth-system processes on ecosystem dynamics but few studies span the inter-decadal timescales required. We, therefore, examined how variation in annual weather patterns associated with the North Atlantic Oscillation (NAO) over four decades was linked to synchrony and stability in a metacommunity of stream invertebrates across multiple, contrasting headwaters in central Wales (UK). Prolonged warmer and wetter conditions during positive NAO winters appeared to synchronize variations in population and community composition among and within streams thereby reducing stability across levels of ecological organization. This climatically mediated synchronization occurred in all streams irrespective of acid-base status and land use, but was weaker where invertebrate communities were more functionally diverse. Wavelet linear models indicated that variation in the NAO explained up to 50% of overall synchrony in species abundances at a timescale of 4-6 years. The NAO appeared to affect ecological dynamics through local variations in temperature, precipitation and discharge, but increasing hydrochemical variability within sites during wetter winters might have contributed. Our findings illustrate how large-scale climatic fluctuations generated over the North Atlantic can affect population persistence and dynamics in inland freshwater ecosystems in ways that transcend local catchment character. Protecting and restoring functional diversity in stream communities might increase their stability against warmer, wetter conditions that are analogues of ongoing climate change. Catchment management could also dampen impacts and provide options for climate change adaptation.

Merging trait-based ecology and regime shift theory to anticipate community responses to warming.

Anthropogenic warming is altering species abundance, distribution, physiology, and more. How changes observed at the species level alter emergent community properties is an active and urgent area of research. Trait-based ecology and regime shift theory provide complementary ways to understand climate change impacts on communities, but these two bodies of work are only rarely integrated. Lack of integration handicaps our ability to understand community responses to warming, at a time when such understanding is critical. Therefore, we advocate for merging trait-based ecology with regime shift theory. We propose a general set of principles to guide this merger and apply these principles to research on marine communities in the rapidly warming North Atlantic. In our example, combining trait distribution and regime shift analyses at the community level yields greater insight than either alone. Looking forward, we identify a clear need for expanding quantitative approaches to collecting and merging trait-based and resilience metrics in order to advance our understanding of climate-driven community change.

North Atlantic jet stream projections in the context of the past 1,250 years.

Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st-century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we incorporate insights from an ensemble of last-millennium water isotope-enabled climate model simulations and a wide array of mean annual water isotope ([Formula: see text]O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.

Cenozoic migration of a desert plant lineage across the North Atlantic.

Previous paleobotanical work concluded that Paleogene elements of the sclerophyllous subhumid vegetation of western Eurasia and western North America were endemic to these disjunct regions, suggesting that the southern areas of the Holarctic flora were isolated at that time. Consequently, molecular studies invoked either parallel adaptation to dry climates from related ancestors, or long-distance dispersal in explaining disjunctions between the two regions, dismissing the contemporaneous migration of dry-adapted lineages via land bridges as unlikely. We report Vauquelinia (Rosaceae), currently endemic to western North America, in Cenozoic strata of western Eurasia. Revision of North American fossils previously assigned to Vauquelinia confirmed a single fossil-species of Vauquelinia and one of its close relative Kageneckia. We established taxonomic relationships of fossil-taxa using diagnostic character combinations shared with modern species and constructed a time-calibrated phylogeny. The fossil record suggests that Vauquelinia, currently endemic to arid and subdesert environments, originated under seasonally arid climates in the Eocene of western North America and subsequently crossed the Paleogene North Atlantic land bridge (NALB) to Europe. This pattern is replicated by other sclerophyllous, dry-adapted and warmth-loving plants, suggesting that several of these taxa potentially crossed the North Atlantic via the NALB during Eocene times.

Overturning in the subpolar North Atlantic: a review.

The Overturning in the Subpolar North Atlantic Program (OSNAP) was initiated in the spring of 2010 through a collaborative effort involving the USA, the UK, Germany, the Netherlands and Canada. A key feature of OSNAP is a trans-basin observing system deployed in the summer of 2014 for the continuous measure of volume, heat and freshwater fluxes in the subpolar North Atlantic. This review focuses on advancements made possible by the collective OSNAP observations. Chief among those advancements is the quantification of the dominant role of the eastern subpolar North Atlantic in the production of dense waters that reside in the lower limb of the overturning: the Irminger and Iceland basins contributed approximately three times as much dense water compared with the Labrador Sea over the observational period. Other advancements include elucidation of the relationship between convective activity in the basin interior and boundary current anomalies; the spread of overflow waters in the subpolar region; the seasonality of the meridional volume, heat and freshwater fluxes; and the challenges involved in designing a simpler, less costly observing system. Collectively, OSNAP measurements are laying a framework on which to assess the overturning circulation's vulnerability to continued warming and freshening as climate change continues apace. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Arctic freshwater impact on the Atlantic Meridional Overturning Circulation: status and prospects.

Arguably, the most conspicuous evidence for anthropogenic climate change lies in the Arctic Ocean. For example, the summer-time Arctic sea ice extent has declined over the last 40 years and the Arctic Ocean freshwater storage has increased over the last 30 years. Coupled climate models project that this extra freshwater will pass Greenland to enter the sub-polar North Atlantic Ocean (SPNA) in the coming decades. Coupled climate models also project that the Atlantic Meridional Overturning Circulation (AMOC) will weaken in the twenty-first century, associated with SPNA buoyancy increases. Yet, it remains unclear when the Arctic anthropogenic freshening signal will be detected in the SPNA, or what form the signal will take. Therefore, this article reviews and synthesizes the state of knowledge on Arctic Ocean and SPNA salinity variations and their causes. This article focuses on the export processes in data-constrained ocean circulation model hindcasts. One challenge is to quantify and understand the relative importance of different competing processes. This article also discusses the prospects to detect the emergence of Arctic anthropogenic freshening and the likely impacts on the AMOC. For this issue, the challenge is to distinguish anthropogenic signals from natural variability. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Long term trends in floating plastic pollution within a marine protected area identifies threats for Endangered northern bottlenose whales.

"The Gully", situated off Nova Scotia, Canada, is the largest submarine canyon in the western North Atlantic. This unique oceanographic feature, which became a Marine Protected Area (MPA) in 2004, is rich in marine biodiversity and is part of the critical habitat of Endangered northern bottlenose whales (Hyperoodon ampullatus). To understand the potential impact of plastic pollution in the MPA and on this Endangered cetacean, we evaluated trends over time in the abundance and composition of plastics and compared these to the stomach contents of recently stranded northern bottlenose whales. From the 1990s-2010s, the median abundance of micro-sized (<5 mm) and small plastics (5 mm-2.5 cm) increased significantly, while the median abundance of large plastics (>2.5 cm) decreased significantly. Plastic abundance from the 2010s for micro-sized and small plastics varied from 5586-438 196 particles km-2, higher than previously measured estimates for surrounding offshore areas. Polymers identified using FTIR spectroscopy included polyethylene, polypropylene, polyethylene terephthalate polyester, nylon, alkyds (paint), and natural and semi-synthetic cellulosic fibers. The abundance of large debris ranged from 0 to 108.6 items km-2 and consisted of plastic sheets and bags, food wrappers and containers, rope, fishing buoys, and small plastic fragments. Whale stomach contents contained fragments of fishing nets, ropes, bottle caps, cups, food wrappers, smaller plastic fragments, fibers, and paint flakes, consistent with the composition and character of items collected from their critical habitat. Despite being far from centres of human population, the unique oceanographic features of The Gully (i.e. currents and bathymetric complexity) may concentrate plastic debris, increasing exposure rates of whales to plastic pollution. The increase in micro-sized and small plastics over time suggests associated health and welfare impacts of ingested plastics should be accounted for in future recovery plans for this Endangered species.

Elevated dust depositions in West Asia linked to ocean-atmosphere shifts during North Atlantic cold events.

Rapid North Atlantic cooling events during the last deglaciation caused atmospheric reorganizations on global and regional scales. Their impact on Asian climate has been investigated for monsoonal domains, but remains largely unknown in westerly wind-dominated semiarid regions. Here we generate a dust record from southeastern Iran spanning the period 19 to 7 cal. ka B.P. We find a direct link between frequent occurrences of dust plumes originating from the Arabian Peninsula and North Africa and rapid southward shifts of the westerlies associated with changes of the winter stationary waves during Heinrich Stadial 1, the Younger Dryas, the Preboreal Oscillation, and the 8.2-ka event. Dust input rises and falls abruptly at the transitions into and out of these cooling events, which we attribute to changes in the ocean circulation strength that are modulated by the North Atlantic winter sea-ice cover. Our findings reveal that waxing and waning of North American ice sheets have a stronger influence than those of European ice sheets on the winter climate over West Asia.

Early atmospheric contamination on the top of the Himalayas since the onset of the European Industrial Revolution.

Because few ice core records from the Himalayas exist, understanding of the onset and timing of the human impact on the atmosphere of the "roof of the world" remains poorly constrained. We report a continuous 500-y trace metal ice core record from the Dasuopu glacier (7,200 m, central Himalayas), the highest drilling site on Earth. We show that an early contamination from toxic trace metals, particularly Cd, Cr, Mo, Ni, Sb, and Zn, emerged at high elevation in the Himalayas at the onset of the European Industrial Revolution (∼1780 AD). This was amplified by the intensification of the snow accumulation (+50% at Dasuopu) likely linked to the meridional displacement of the winter westerlies from 1810 until 1880 AD. During this period, the flux and crustal enrichment factors of the toxic trace metals were augmented by factors of 2 to 4 and 2 to 6, respectively. We suggest this contamination was the consequence of the long-range transport and wet deposition of fly ash from the combustion of coal (likely from Western Europe where it was almost entirely produced and used during the 19th century) with a possible contribution from the synchronous increase in biomass burning emissions from deforestation in the Northern Hemisphere. The snow accumulation decreased and dry winters were reestablished in Dasuopu after 1880 AD when lower than expected toxic metal levels were recorded. This indicates that contamination on the top of the Himalayas depended primarily on multidecadal changes in atmospheric circulation and secondarily on variations in emission sources during the last 200 y.

Iceland as Stepping Stone for Spread of Highly Pathogenic Avian Influenza Virus between Europe and North America.

Highly pathogenic avian influenza viruses (HPAIVs) of hemagglutinin type H5 and clade 2.3.4.4b have widely spread within the northern hemisphere since 2020 and threaten wild bird populations, as well as poultry production. We present phylogeographic evidence that Iceland has been used as a stepping stone for HPAIV translocation from northern Europe to North America by infected but mobile wild birds. At least 2 independent incursions of HPAIV H5N1 clade 2.3.4.4b assigned to 2 hemagglutinin clusters, B1 and B2, are documented for summer‒autumn 2021 and spring 2022. Spread of HPAIV H5N1 to and among colony-breeding pelagic avian species in Iceland is ongoing. Potentially devastating effects (i.e., local losses >25%) on these species caused by extended HPAIV circulation in space and time are being observed at several affected breeding sites throughout the North Atlantic.

Climate oscillations drive millennial-scale changes in seabird colony size.

Seabird population size is intimately linked to the physical, chemical, and biological processes of the oceans. Yet, the overall effects of long-term changes in ocean dynamics on seabird colonies are difficult to quantify. Here, we used dated lake sediments to reconstruct ~10,000-years of seabird dynamics in the Northwest Atlantic to determine the influences of Holocene-scale climatic oscillations on colony size. On Baccalieu Island (Newfoundland and Labrador, Canada)-where the world's largest colony of Leach's storm-petrel (Hydrobates leucorhous Vieillot 1818) currently breeds-our data track seabird colony growth in response to warming during the Holocene Thermal Maximum (ca. 9000 to 6000 BP). From ca. 5200 BP to the onset of the Little Ice Age (ca. 550 BP), changes in colony size were correlated to variations in the North Atlantic Oscillation (NAO). By contrasting the seabird trends from Baccalieu Island to millennial-scale changes of storm-petrel populations from Grand Colombier Island (an island in the Northwest Atlantic that is subjected a to different ocean climate), we infer that changes in NAO influenced the ocean circulation, which translated into, among many things, changes in pycnocline depth across the Northwest Atlantic basin where the storm-petrels feed. We hypothesize that the depth of the pycnocline is likely a strong bottom-up control on surface-feeding storm-petrels through its influence on prey accessibility. Since the Little Ice Age (LIA), the effects of ocean dynamics on seabird colony size have been altered by anthropogenic impacts. Subsequently, the colony on Baccalieu Island grew at an unprecedented rate to become the world's largest resulting from favorable conditions linked to climate warming, increased vegetation (thereby nesting habitat), and attraction of recruits from other colonies that are now in decline. We show that although ocean dynamics were an important driver of seabird colony dynamics, its recent influence has been modified by human interference.

Strong and lasting impacts of past global warming on baleen whales and their prey.

Global warming is affecting the population dynamics and trophic interactions across a wide range of ecosystems and habitats. Translating these real-time effects into their long-term consequences remains a challenge. The rapid and extreme warming period that occurred after the Last Glacial Maximum (LGM) during the Pleistocene-Holocene transition (7-12 thousand years ago) provides an opportunity to gain insights into the long-term responses of natural populations to periods with global warming. The effects of this post-LGM warming period have been assessed in many terrestrial taxa, whereas insights into the impacts of rapid global warming on marine taxa remain limited, especially for megafauna. In order to understand how large-scale climate fluctuations during the post-LGM affected baleen whales and their prey, we conducted an extensive, large-scale analysis of the long-term effects of the post-LGM warming on abundance and inter-ocean connectivity in eight baleen whale and seven prey (fish and invertebrates) species across the Southern and the North Atlantic Ocean; two ocean basins that differ in key oceanographic features. The analysis was based upon 7032 mitochondrial DNA sequences as well as genome-wide DNA sequence variation in 100 individuals. The estimated temporal changes in genetic diversity during the last 30,000 years indicated that most baleen whale populations underwent post-LGM expansions in both ocean basins. The increase in baleen whale abundance during the Holocene was associated with simultaneous changes in their prey and climate. Highly correlated, synchronized and exponential increases in abundance in both baleen whales and their prey in the Southern Ocean were indicative of a dramatic increase in ocean productivity. In contrast, the demographic fluctuations observed in baleen whales and their prey in the North Atlantic Ocean were subtle, varying across taxa and time. Perhaps most important was the observation that the ocean-wide expansions and decreases in abundance that were initiated by the post-LGM global warming, continued for millennia after global temperatures stabilized, reflecting persistent, long-lasting impacts of global warming on marine fauna.

Decadal-scale phenology and seasonal climate drivers of migratory baleen whales in a rapidly warming marine ecosystem.

Species' response to rapid climate change can be measured through shifts in timing of recurring biological events, known as phenology. The Gulf of Maine is one of the most rapidly warming regions of the ocean, and thus an ideal system to study phenological and biological responses to climate change. A better understanding of climate-induced changes in phenology is needed to effectively and adaptively manage human-wildlife conflicts. Using data from a 20+ year marine mammal observation program, we tested the hypothesis that the phenology of large whale habitat use in Cape Cod Bay has changed and is related to regional-scale shifts in the thermal onset of spring. We used a multi-season occupancy model to measure phenological shifts and evaluate trends in the date of peak habitat use for North Atlantic right (Eubalaena glacialis), humpback (Megaptera novaeangliae), and fin (Balaenoptera physalus) whales. The date of peak habitat use shifted by +18.1 days (0.90 days/year) for right whales and +19.1 days (0.96 days/year) for humpback whales. We then evaluated interannual variability in peak habitat use relative to thermal spring transition dates (STD), and hypothesized that right whales, as planktivorous specialist feeders, would exhibit a stronger response to thermal phenology than fin and humpback whales, which are more generalist piscivorous feeders. There was a significant negative effect of western region STD on right whale habitat use, and a significant positive effect of eastern region STD on fin whale habitat use indicating differential responses to spatial seasonal conditions. Protections for threatened and endangered whales have been designed to align with expected phenology of habitat use. Our results show that whales are becoming mismatched with static seasonal management measures through shifts in their timing of habitat use, and they suggest that effective management strategies may need to alter protections as species adapt to climate change.

Anthropogenic Aerosols Modulated 20th-Century Sahel Rainfall Variability Via Their Impacts on North Atlantic Sea Surface Temperature.

The Sahel rainfall has a close teleconnection with North Atlantic sea surface temperature (NASST) variability, which has separately been shown to be affected by aerosols. Therefore, changes in regional aerosols emission could potentially drive multidecadal Sahel rainfall variability. Here we combine ensembles of state-of-the-art global climate models (the CESM and CanESM large ensemble simulations and CMIP6 models) with observational data sets to demonstrate that anthropogenic aerosols have significantly impacted 20th-century detrended Sahel rainfall multidecadal variability through modifying NASST. We show that aerosol-induced multidecadal variations of downward solar radiative fluxes over the North Atlantic cause NASST variability during the 20th century, altering the ITCZ position and dynamically linking aerosol effects to Sahel rainfall variability. This process chain is caused by aerosol-induced changes in radiative surface fluxes rather than changes in ocean circulations. CMIP6 models further suggest that aerosol-cloud interactions modulate the inter-model uncertainty of simulated NASST and potentially the Sahel rainfall variability.

Freshwater Flux Variability Lengthens the Period of the Low-Frequency AMOC Variability.

Atlantic Meridional Overturning Circulation (AMOC) exhibits interdecadal to multidecadal variability, yet the role of surface freshwater flux (FWF) variability in this AMOC variability remains unclear. This study isolates the contribution of FWF variability in modulating AMOC through a partially coupled experiment, in which the effect of the interactive FWF is disabled. It is demonstrated that the impact of the coupled FWF variability enhances the persistence of density and deep convection anomalies in the Labrador Sea (LS), thus lengthening the period of the AMOC oscillation on multidecadal timescale and suppressing its ∼30-year periodicity. Further lead-lag regressions illuminate that the more persistent LS density anomalies are maintained by two mechanisms: (a) The local temperature-salinity coupling through the evaporation and (b) a downstream propagation along the East Greenland Current of the extra salinity anomaly due to the sea ice melting changes associated with an atmosphere forcing over the southern Greenland tip.

Growth Resilience of Subarctic Rhodoliths (Lithothamnion glaciale, Rhodophyta) to Chronic Low Sea Temperature and irradiance.

Rhodolith beds are pervasive marine biological systems in the subarctic North Atlantic. Limited knowledge about effects of temperature and irradiance on rhodolith growth limits the ability to anticipate the response of rhodolith beds to this ocean's chronic low, yet changing sea temperature and irradiance regimes. We carried out a 149-d laboratory experiment with Newfoundland Lithothamnion glaciale rhodoliths to test the predictions that growth (i) is inhibited at temperatures of ~0.5°C and (ii) resumes as temperature increases above 0.5°C, albeit at a higher rate under high than low irradiances. Rhodoliths were grown in experimental tanks at near-zero (~0.7°C) seawater temperatures during the first 85 d and at temperatures increasing naturally to ~6°C for the remaining 64 d. Rhodoliths in those tanks were exposed to either low (0.02 mol photons·m-2 ·d-1 ) or high (0.78 mol photons·m-2 ·d-1 ) irradiances during the entire experiment. Rhodoliths grew at a linear rate of ~281 µm·year-1 (0.77 µm·d-1 ) throughout the experiment under both irradiance treatments despite daily seawater temperature variation of up to 3°C. Near-zero temperatures of ~0.5 to 1.0°C did not inhibit rhodolith growth. Model selection showed that PAR-day (a cumulative irradiance index) was a better predictor of growth variation than Degree-day (a cumulative thermal index). Our findings extend to ~0.5°C the lower limit of the known temperature range (~1 to at least 16°C) over which growth in L. glaciale rhodoliths remains unaffected, while suggesting that the growth-irradiance relationship in low-light environments at temperatures below 6°C is less irradiance-driven than recently proposed.

Predictable hydrological and ecological responses to Holocene North Atlantic variability.

Climate variations in the North Atlantic region can substantially impact surrounding continents. Notably, the Younger Dryas chronozone was named for the ecosystem effects of abrupt changes in the region at circa (ca.) 12.9-11.7 ka (millennia before 1950 AD). Holocene variations since then, however, have been hard to diagnose, and the responsiveness of terrestrial ecosystems continues to be debated. Here, we show that Holocene climate variations had spatial patterns consistent with changes in Atlantic overturning and repeatedly steepened the temperature gradient between Nova Scotia and Greenland since >8 ka. The multicentury changes correlated with hydrologic and vegetation changes in the northeast United States, including when an enhanced temperature gradient coincided with subregional droughts indicated by water-level changes at multiple coastal lakes at 4.9-4.6, 4.2-3.9, 2.8-2.1, and 1.3-1.2 ka. We assessed the variability and its effects by replicating signals across sites, using converging evidence from multiple methods, and applying forward models of the systems involved. We evaluated forest responses in the northeast United States and found that they tracked the regional climate shifts including the smallest magnitude (∼5% or 50 mm) changes in effective precipitation. Although a long-term increase in effective precipitation of >45% (>400 mm) could have prevented ecological communities from equilibrating to the continuously changing conditions, our comparisons confirm stable vegetation-climate relationships and support the use of fossil pollen records for quantitative paleoclimate reconstruction. Overall, the network of records indicates that centennial climate variability has repeatedly affected the North Atlantic region with predictable consequences.

Two deep-mantle sources for Paleocene doming and volcanism in the North Atlantic.

The North Atlantic Igneous Province (NAIP) erupted in two major pulses that coincide with the continental breakup and the opening of the North Atlantic Ocean over a period from 62 to 54 Ma. The unknown mantle structure under the North Atlantic during the Paleocene represents a major missing link in deciphering the geodynamic causes of this event. To address this outstanding challenge, we use a back-and-forth iterative method for time-reversed global convection modeling over the Cenozoic Era which incorporates models of present-day tomography-based mantle heterogeneity. We find that the Paleocene mantle under the North Atlantic is characterized by two major low-density plumes in the lower mantle: one beneath Greenland and another beneath the Azores. These strong lower-mantle upwellings generate small-scale hot upwellings and cold downwellings in the upper mantle. The upwellings are dispersed sources of magmatism and topographic uplift that were active on the rifted margins of the North Atlantic during the formation of the NAIP. While most studies of the Paleocene evolution of the North Atlantic have focused on the proto-Icelandic plume, our Cenozoic reconstructions reveal the equally important dynamics of a hot, buoyant, mantle-wide upwelling below the Azores.