High-latitude North Atlantic Ocean Research Articles

Tracing anthropogenic aerosol trace metal sources in the North Atlantic Ocean using Pb, Zn and Ni isotopes

Atmospheric deposition of trace metals of natural or anthropogenic origin is an important input of micronutrients to the surface ocean. However, understanding its direct impact on oceanic element cycles is challenging due to scarce data, coupled to diverse aerosol sources and variable solubilities. Here, we present a dataset that combines Ni, Zn and Pb isotopes for samples from the Moroccan and Senegalese coasts and in the high latitude North Atlantic Ocean. We combine the new with published data for other circum-North Atlantic sources to assess the processes that determine the isotope signatures in different types of aerosols. We then use open marine aerosol data to investigate the impact of these signatures in the open ocean. Isotope analyses were conducted on bulk aerosols (TSP), on their ultra-high-purity water leachates, and on rainwaters. Aerosols characterized by crustal elemental abundances have isotope compositions similar to Saharan mineral dust. Mixing with anthropogenic aerosols from Europe/North Africa results in lower 206Pb/207Pb and 208Pb/207Pb values for the Eastern North Atlantic region. Higher 206Pb/207Pb at a given 208Pb/207Pb, observed near the Canadian margin and occasionally at the Senegalese coast, points to anthropogenic inputs from North America. Based on trends in the aerosol data (e.g., δ66ZnJMC-Lyon versus 206Pb/207Pb, δ60NiSRM986 versus Ni/V), we identify several anthropogenic sources of Zn and Ni. The δ66ZnJMC-Lyon of low-temperature pollution (e.g., non-exhaust traffic emission) appears to be around −0.1‰ to 0.2‰, while leachate δ66ZnJMC-Lyon as low as −0.21‰ indicates contributions from high-temperature combustion or smelting processes. Among aerosols with good correlations between Ni and V, δ60NiSRM986 > 0.40‰ traces Ni contributions from oil combustion. Other Ni-enriched sources, possibly originating from laterite or sulfide, show relatively low δ60NiSRM986 (as low as −0.85‰) and low V/Ni. Generally, aerosol sources for Zn are consistent throughout the North Atlantic, while Ni can be highly heterogenous. Combining the new data with literature elemental data, ratios of soluble Zn/Pb in anthropogenic aerosols are 1–100 times surface ocean ratios, suggesting that the low δ66ZnJMC-Lyon observed in anthropogenic aerosol can be key in controlling the upper ocean Zn isotope composition. These aerosols have, however, much less significance for surface ocean Ni.

Biogeographic characterisation of blue whale song worldwide: using song to identify populations

Blue whale songs provide a measure for characterising worldwide blue whale population structure. These songs are divided into nine regional types, which maintain a stable character. Five of the nine song types have been recorded over time spans greater than 30 years showing no significant change in character. The nine song types can be divided into those containing only simple tonal components (high latitude North Pacific, North Atlantic and Southern Ocean song types), those comprised of complex pulsed units in addition to the tonal components (Pacific Ocean margin song types from California, Chile and New Zealand), and those which have the greatest complexity of all and the longest cycling times (Indian Ocean song types from Sri Lanka, Fremantle and Diego Garcia). We suggest that temporally stable differences in song provide another characteristic for comparison with genetic and morphological data when defining blue whale populations. Furthermore, as Mellinger and Barlow (2003) recommend, when there is a lack of other data or lack of clarity in other data sets, evidence of distinct differences in songs between areas should be used as a provisional hypothesis about population structure when making management decisions. Worldwide study is needed to better understand the various populations and subspecies within species like the blue whale that have large geographic distributions and have both migrating and resident populations.

Effect of Plankton Composition Shifts in the North Atlantic on Atmospheric pCO2

AbstractMarine carbon cycle processes are important for taking up atmospheric CO2 thereby reducing climate change. Net primary and export production are important pathways of carbon from the surface to the deep ocean where it is stored for millennia. Climate change can interact with marine ecosystems via changes in the ocean stratification and ocean circulation. In this study we use results from the Community Earth System Model version 2 (CESM2) to assess the effect of a changing climate on biological production and phytoplankton composition in the high latitude North Atlantic Ocean. We find a shift in phytoplankton type dominance from diatoms to small phytoplankton which reduces net primary and export productivity. Using a conceptual carbon‐cycle model forced with CESM2 results, we give a rough estimate of a positive phytoplankton composition‐atmospheric CO2 feedback of approximately 60 GtCO2/°C warming in the North Atlantic which lowers the 1.5° and 2.0°C warming safe carbon budgets.

Multi-decadal to centennial scales variability in the East Asian Summer Monsoon around the 5.5 kyr B.P. climate event

Understanding the variability of the mid-Holocene climate on multi-decadal to centennial scales, including the well-known 5.5 kyr B.P. cold event, is important to better evaluate possible changes of the East Asian Summer Monsoon (EASM) as global warming progresses. Over the past few decades, stalagmite oxygen isotope records have been widely used as a proxy for the EASM; however, most high-resolution stalagmite records are from southern China. In this study, a 4-yr-resolution δ18O record obtained from stalagmite TW704 in northeastern China was used to reconstruct regional precipitation and the EASM between 5.82 and 4.77 kyr B.P. (where B.P. indicates “before 1950 A.D.”). In general, the inferred EASM variations agree with previously published stalagmite records from southwestern and central China, suggesting synchronous variations of the EASM on decadal to centennial scales across monsoonal China. Our new record also features relatively large decadal–centennial EASM oscillations from northern China, with two centennial-scale weak EASM events centred at 5.6 and 5.0 kyr B.P. The former corresponds to the well-known 5.5 kyr B.P. cold event that occurred in the high-latitude North Atlantic Ocean. These depressed EASM events were probably caused by prolonged periods of El Niño, resulting in decreased precipitation over northeastern China. Our study provides robust evidence to support the hypothesis that ENSO played an important role in modulating the EASM during the mid-Holocene.

Arctic – Atlantic Exchange of the Dissolved Micronutrients Iron, Manganese, Cobalt, Nickel, Copper and Zinc With a Focus on Fram Strait

AbstractThe Arctic Ocean is considered a source of micronutrients to the Nordic Seas and the North Atlantic Ocean through the gateway of Fram Strait (FS). However, there is a paucity of trace element data from across the Arctic Ocean gateways, and so it remains unclear how Arctic and North Atlantic exchange shapes micronutrient availability in the two ocean basins. In 2015 and 2016, GEOTRACES cruises sampled the Barents Sea Opening (GN04, 2015) and FS (GN05, 2016) for dissolved iron (dFe), manganese (dMn), cobalt (dCo), nickel (dNi), copper (dCu) and zinc (dZn). Together with the most recent synopsis of Arctic‐Atlantic volume fluxes, the observed trace element distributions suggest that FS is the most important gateway for Arctic‐Atlantic dissolved micronutrient exchange as a consequence of Intermediate and Deep Water transport. Combining fluxes from FS and the Barents Sea Opening with estimates for Davis Strait (GN02, 2015) suggests an annual net southward flux of 2.7 ± 2.4 Gg·a−1 dFe, 0.3 ± 0.3 Gg·a−1 dCo, 15.0 ± 12.5 Gg·a−1 dNi and 14.2 ± 6.9 Gg·a−1 dCu from the Arctic toward the North Atlantic Ocean. Arctic‐Atlantic exchange of dMn and dZn were more balanced, with a net southbound flux of 2.8 ± 4.7 Gg·a−1 dMn and a net northbound flux of 3.0 ± 7.3 Gg·a−1 dZn. Our results suggest that ongoing changes to shelf inputs and sea ice dynamics in the Arctic, especially in Siberian shelf regions, affect micronutrient availability in FS and the high latitude North Atlantic Ocean.

Mediterranean heat injection to the North Atlantic delayed the intensification of Northern Hemisphere glaciations

The intensification of Northern Hemisphere glaciations at the end of the Pliocene epoch marks one of the most substantial climatic shifts of the Cenozoic. Despite global cooling, sea surface temperatures in the high latitude North Atlantic Ocean rose between 2.9–2.7 million years ago. Here we present sedimentary geochemical proxy data from the Gulf of Cadiz to reconstruct the variability of Mediterranean Outflow Water, an important heat source to the North Atlantic. We find evidence for enhanced production of Mediterranean Outflow from the mid-Pliocene to the late Pliocene which we infer could have driven a sub-surface heat channel into the high-latitude North Atlantic. We then use Earth System Models to constrain the impact of enhanced Mediterranean Outflow production on the northward heat transport in the North Atlantic. In accord with the proxy data, the numerical model results support the formation of a sub-surface channel that pumped heat from the subtropics into the high latitude North Atlantic. We further suggest that this mechanism could have delayed ice sheet growth at the end of the Pliocene.

Impacts of Climate Change on Methylmercury Formation and Bioaccumulation in the 21st Century Ocean

Climate change is altering marine biogeochemistry, thereby modifying factors affecting the formation and trophic transfer of the bioaccumulative neurotoxicant methylmercury (MeHg) in the global ocean. However, both the directionality and magnitude of future changes are unclear. Here we use a coupled 3-D atmosphere-ocean-ecology model to forecast how MeHg concentrations in the surface ocean will respond to changes in primary production and associated carbon export in 2100. Model results suggest a 68-93% increase in MeHg in the polar oceans and a 23% decrease in the high-latitude North Atlantic Ocean. Phytoplankton MeHg reservoirs will increase at high-latitudes and decrease in the mid- and low-latitude oceans due to a poleward shift in habitat. Ocean acidification is projected to increase phytoplankton MeHg reservoirs globally by promoting the growth of Synechococcus, a small phytoplankton species that efficiently accumulates MeHg. Changes in MeHg reservoirs in herbivorous zooplankton mimic their prey but with larger magnitudes due to non-linearity embedded in the food web structure. The effect of climate-driven shifts in marine biogeochemistry thus needs to be considered in evaluating future trajectories in biological MeHg concentrations.

The eastern extent of seasonal iron limitation in the high latitude North Atlantic Ocean

The availability of iron (Fe) can seasonally limit phytoplankton growth in the High Latitude North Atlantic (HLNA), greatly reducing the efficiency of the biological carbon pump. However, the spatial extent of seasonal iron limitation is not yet known. We present autumn nutrient and dissolved Fe measurements, combined with microphytoplankton distribution, of waters overlying the Hebridean (Scottish) shelf break. A distinct biogeochemical divide was observed, with Fe deficient surface waters present beyond the shelf break, much further eastwards than previously recognised. Due to along and on-shelf circulation, the Hebridean shelf represents a much-localised source of Fe, which does not fertilise the wider HLNA. Shelf sediments are generally thought to supply large quantities of Fe to overlying waters. However, for this Fe to influence upper-ocean biogeochemical cycling, efficient off-shelf transport mechanisms are required. This work challenges the view that the oceanic surface waters in close proximity to continental margins are iron replete with respect to marine primary production demands.

Characterization of a Time-Domain Dual Lifetime Referencing pCO2 Optode and Deployment as a High-Resolution Underway Sensor across the High Latitude North Atlantic Ocean

The ocean is a major sink for anthropogenic carbon dioxide (CO2), with the CO2 uptake causing changes to ocean chemistry. To monitor these changes and provide a chemical background for biological and biogeochemical studies, high quality partial pressure of CO2 (pCO2) sensors are required, with suitable accuracy and precision for ocean measurements. Optodes have the potential to measure in situ pCO2 without the need for wet chemicals or bulky gas equilibration chambers that are typically used in pCO2 systems. However, optodes are still in an early developmental stage compared to more established equilibrator-based pCO2 systems. In this study, we performed a laboratory-based characterization of a time-domain dual lifetime referencing pCO2 optode system. The pCO2 optode spot was illuminated with low intensity light (0.2 mA, 0.72 mW) to minimize spot photobleaching. The spot was calibrated using an experimental gas calibration rig prior to deployment, with a determined response time (τ63) of 50 s at 25°C. The pCO2 optode was deployed as an autonomous shipboard underway system across the high latitude North Atlantic Ocean with a resolution of ca.10 measurements per hour. The optode data was validated with a secondary shipboard equilibrator-based infrared pCO2 instrument, and pCO2 calculated from discrete samples of dissolved inorganic carbon and total alkalinity. Further verification of the pCO2 optode data was achieved using complimentary variables such as nutrients and dissolved oxygen. The shipboard precision of the pCO2 sensor was 9.5 μatm determined both from repeat measurements of certified reference materials and from the standard deviation of seawater measurements while on station. Finally, the optode deployment data was used to evaluate the physical and biogeochemical controls on pCO2.

Episodic release of CO2 from the high-latitude North Atlantic Ocean during the last 135\u2009kyr

Antarctic ice cores document glacial-interglacial and millennial-scale variability in atmospheric pCO2 over the past 800 kyr. The ocean, as the largest active carbon reservoir on this timescale, is thought to have played a dominant role in these pCO2 fluctuations, but it remains unclear how and where in the ocean CO2 was stored during glaciations and released during (de)glacial millennial-scale climate events. The evolution of surface ocean pCO2 in key locations can therefore provide important clues for understanding the ocean’s role in Pleistocene carbon cycling. Here we present a 135-kyr record of shallow subsurface pCO2 and nutrient levels from the Norwegian Sea, an area of intense CO2 uptake from the atmosphere today. Our results suggest that the Norwegian Sea probably acted as a CO2 source towards the end of Heinrich stadials HS1, HS4 and HS11, and may have contributed to the increase in atmospheric pCO2 at these times.

Geochemical response of the mid-depth Northeast Atlantic Ocean to freshwater input during Heinrich events 1 to 4

Heinrich events are intervals of rapid iceberg-sourced freshwater release to the high latitude North Atlantic Ocean that punctuate late Pleistocene glacials. Delivery of fresh water to the main North Atlantic sites of deep water formation during Heinrich events may result in major disruption to the Atlantic Meridional Overturning Circulation (AMOC), however, the simple concept of an AMOC shutdown in response to each freshwater input has recently been shown to be overly simplistic. Here we present a new multi-proxy dataset spanning the last 41,000 years that resolves four Heinrich events at a classic mid-depth North Atlantic drill site, employing four independent geochemical tracers of water mass properties: boron/calcium, carbon and oxygen isotopes in foraminiferal calcite and neodymium isotopes in multiple substrates. We also report rare earth element distributions to investigate the fidelity by which neodymium isotopes record changes in water mass distribution in the northeast North Atlantic. Our data reveal distinct geochemical signatures for each Heinrich event, suggesting that the sites of fresh water delivery and/or rates of input played at least as important a role as the stage of the glacial cycle in which the fresh water was released. At no time during the last 41 kyr was the mid-depth northeast North Atlantic dominantly ventilated by southern-sourced water. Instead, we document persistent ventilation by Glacial North Atlantic Intermediate Water (GNAIW), albeit with variable properties signifying changes in supply from multiple contributing northern sources.

An Interdecadal Increase in the Spring Bering Sea Ice Cover in 2007

The sea ice coverage of the Northern Hemisphere as a whole has been declining since 1979. On contrary, the March-April sea ice concentration in the Bering Sea experienced a prominent increase in year 2007. The present study documents the changes in surface air temperature, surface heat fluxes, sea surface temperature, and atmospheric circulation accompanying the above interdecadal change in the Bering Sea ice concentration. It is shown that an obvious decrease in surface air temperature, sea surface temperature, and surface net shortwave radiation occurred in concurrent with the sea ice increase. The surface air temperature decrease is associated with a large-scale circulation change, featuring a decrease in sea level pressure extending from the Pacific coast of Alaska to northwestern Europe and an increase in sea level pressure over the high-latitude Asia and the high-latitude North Atlantic Ocean. The enhancement of northwesterly winds over the Bering Sea led to a large decrease in surface air temperature there. The associated increase in upward turbulent heat flux cooled the sea surface temperature in the waters south of the ice covered region, favoring the southward expansion of ice extent. This, together with a positive ice-albedo feedback, amplified the sea ice anomalies after they were initiated, leading to the interdecadal increase in sea ice in the Bering Sea.

Paleomagnetic field variability and chronostratigraphy of Brunhes-Chron deep-sea sediments from the Bering Sea: IODP Expedition 323

IODP Expedition 323 recovered six complete and replicate records of Brunhes-Chron paleomagnetic field variability (0–780,000 years BP) in 2820m core depth below sea floor (CSF) of deep-sea sediments. On shipboard, we made more than 220,000 paleomagnetic measurements on the recovered sediments. Since then, we have u-channel sampled more than 300m of Brunhes Chron sediments to corroborate our shipboard measurements and improve our paleomagnetic and rock magnetic understanding of these sediments. Several intervals of distinctive paleomagnetic secular variation (PSV) have been identified that appear to be correlatable among sites 1343, 1344, and 1345. One magnetic field excursion is recorded in sediments of sites 1339, 1343, 1344, and 1345. We identify this to be excursion 7α/Iceland Basin Event (192,000 years BP), which is also seen in the high-latitude North Atlantic Ocean (Channell et al., 1997). We have verified in u-channels the placement of the Brunhes/Matuyama boundary (780,000 years BP) at sites 1341 and 1343. Finally, we have developed a medium-quality relative paleointensity record for these sediments that is correlatable among the sites, even though it is still biased by large-amplitude environmental variability. On the basis of these observations we have built a magnetic chronostratigraphy of Expedition 323 sediments suitable for regional correlation and dating over the last 1 million years, and compared this with oxygen-isotope chronostratigraphy from sites U1339 and U1345.

Multimodel Assessment of Regional Surface Temperature Trends: CMIP3 and CMIP5 Twentieth-Century Simulations

AbstractRegional surface temperature trends from phase 3 of the Coupled Model Intercomparison Project (CMIP3) and CMIP5 twentieth-century runs are compared with observations—at spatial scales ranging from global averages to individual grid points—using simulated intrinsic climate variability from preindustrial control runs to assess whether observed trends are detectable and/or consistent with the models' historical run trends. The CMIP5 models are also used to detect anthropogenic components of the observed trends, by assessing alternative hypotheses based on scenarios driven with either anthropogenic plus natural forcings combined, or with natural forcings only. Modeled variability is assessed via inspection of control run time series, standard deviation maps, spectral analyses, and low-frequency variance consistency tests. The models are found to provide plausible representations of internal climate variability, although there is room for improvement. The influence of observational uncertainty on the trends is assessed and is found to be generally small in comparison with intrinsic climate variability. Observed temperature trends over 1901–2010 are found to contain detectable anthropogenic warming components over a large fraction (about 80%) of the analyzed global area. In about 70% of the analyzed area, the modeled warming is consistent with the observed trends; in about 10% it is significantly greater than simulated. Regions without detectable warming include the high-latitude North Atlantic Ocean, the eastern United States, and parts of the eastern and northern Pacific Ocean. For 1981–2010, the observed warming trends over only about 30% of the globe are found to contain a detectable anthropogenic warming: this includes a number of regions within about 40°–45° of the equator, particularly in the Indian Ocean, western Pacific, South Asia, and tropical Atlantic.

Spatial and temporal development of phytoplankton iron stress in relation to bloom dynamics in the high‐latitude North Atlantic Ocean

The high‐latitude North Atlantic (HLNA) is characterized by a marked seasonal phytoplankton bloom, which removes the majority of surface macronutrients. However, incomplete nitrate depletion is frequently observed during summer in the region, potentially reflecting the seasonal development of an iron (Fe) limited phytoplankton community. In order to investigate the seasonal development and spatial extent of iron stress in the HLNA, nutrient addition experiments were performed during the spring (May) and late summer (July and August) of 2010. Grow‐out experiments (48–120 h) confirmed the potential for iron limitation in the region. Short‐term (24 h) incubations further enabled high spatial coverage and mapping of phytoplankton physiological responses to iron addition. The difference in the apparent maximal photochemical yield of photosystem II (PSII) (Fv : Fm) between nutrient (iron) amended and control treatments (Δ(Fv : Fm)) was used as a measure of the relative degree of iron stress. The combined observations indicated variability in the seasonal cycle of iron stress between different regions of the Irminger and Iceland Basins of the HLNA, related to the timing of the annual bloom cycle in contrasting biogeochemical provinces. Phytoplankton iron stress developed during the transition from the prebloom to peak bloom conditions in the HLNA and was more severe for larger cells. Subsequently, iron stress was reduced in regions where macronutrients were depleted following the bloom. Iron availability plays a significant role in the biogeochemistry of the HLNA, potentially lowering the efficiency of one of the strongest biological carbon pumps in the ocean.

Cold-water coral ecosystem (Tisler Reef, Norwegian Shelf) may be a hotspot for carbon cycling

Cold-water coral (CWC) reefs are recognised as an important marine benthic eco system at continental margins. Where abundant, they most likely play a role both in the maintenance of biodiversity and in the provision of ecosystem services provided by shelf seas. Here, we directly measure the community respiration of a CWC reef on the Norwegian shelf and show that rates of oxygen uptake are high in summer (25-88 mmol O-2 m(-2) d(-1)), with lower spring values (7.5-9 mmol O-2 m(-2) d(-1)). Spring uptake rates are comparable to values at shelf sediments at similar water depths, while summer values are much higher. We calculate that with such high respiration rates, CWC reef ecosystems potentially turnover a significant proportion (similar to 25%) of the annual shelf carbon export in the Norwegian Sea, where CWC reefs are abundant. The loss of CWCs through physical destruction or ocean acidification may have a significant impact on local carbon cycling in the mid-to high-latitude North Atlantic Ocean.

Reinterpretation of two diatom species from the West Greenland margin — Thalassiosira kushirensis and Thalassiosira antarctica var. borealis — hydrological consequences

Abstract Two representatives of the planktonic diatom genus Thalassiosira , commonly associated together in the moderate to high latitude North Atlantic Ocean, are demonstrated to be significantly different in both morphology and autecology. Thalassiosira antarctica Comber var. borealis Fryxell, Doucette et Hubbard (mostly resting spores) is a dominant species of the early spring phytoplankton bloom around Greenland and in the Arctic. In most paleoclimatic/paleoenvironmental studies of the last decades, Thalassiosira kushirensis Takano resting spores (r.s.) have probably been included in T. antarctica var. borealis . The present study of phytoplankton (from CTD probes and a single surface sediment sample, from a multi core) and comparative analyses of piston cores from the West Greenland margin show these taxa to be good indicators of environmental and hydrological changes related to the distribution of water masses in the studied area. T. antarctica var. borealis is interpreted as an indicator of northern cold water influx (especially increased meltwater input during spring bloom), whereas T. kushirensis r.s. appears to be related to temperate waters of Atlantic origin, and probably reflects reduced meltwater influx and increased ocean forcing. An abundance of T. kushirensis r.s. during the Little Ice Age (LIA) is interpreted to reflect an enhanced influence of the West Greenland Current (WGC) in this region.

Multi-year predictability of the tropical Atlantic atmosphere driven by the high latitude North Atlantic Ocean

[1] Using idealised model experiments we show that the tropical Atlantic main hurricane development region (MDR) is potentially one of the most predictable regions for atmospheric variables such as precipitation and wind shear on multi-year timescales. Similarly we also find predictability for the number of tropical storms and the position of the inter-tropical convergence zone. Further experiments that withhold data in different parts of the ocean identify the North Atlantic sub-polar gyre as the key region for driving the skill in the model MDR. This further highlights the importance of observing the high-latitude North Atlantic Ocean for initialising future decadal predictions.

Radiocarbon reservoir age of high latitude North Atlantic surface water during the last deglacial

The radiocarbon reservoir age of high latitude North Atlantic Ocean surface water is essential for linking the continental and marine climate records, and is expected to vary according to changes in North Atlantic deep water (NADW) production. Measurements from this region also provide important input and/or tests of oceanic radiocarbon using 3-D global ocean circulation models. Here, we present a surface water radiocarbon reservoir age record of the high latitude western North Atlantic for the deglacial period via the use of fossil cold-water corals growing in waters that are rapidly exchanged with nearby surface waters. The reservoir age of high latitude North Atlantic surface waters was computed from the radiocarbon age difference between our radiocarbon calibration record ( http://radiocarbon.LDEO.columbia.edu) and our marine radiocarbon data. 230Th/ 234U/ 238U dates provide the absolute coral ages. Our high latitude North Atlantic Ocean reservoir age data combined with recalculated reservoir ages based on published coexisting terrestrial and marine material and Vedde ash radiocarbon dates from central and eastern North Atlantic show modern values (380±140 year, n=14) during the Bolling and Allerod warm period and a 200 year increase in reservoir age (590±130 year, n=10) during the entire Younger Dryas (YD) cold episode. The reservoir age then decreased to 270±20 year ( n=2) at the Preboreal/YD transition, although the dates are too sparse for us to be confident in this estimate. We are not able to resolve the timing of the transition to increased reservoir ages from the mid-Allerod to the YD due to the relatively small change and correspondingly large uncertainty in the estimates. The atmospheric Δ 14C record derived from our atmospheric radiocarbon record displays a 40 per mil increase from 12,900 to 12,650 cal years BP, coincident with the shift to high reservoir ages in the early YD cold event. Intrusion of 14C depleted Antarctic Intermediate Water (AAIW) to the high latitude North Atlantic and reduction of NADW formation are possible causes for the coincident shift to high reservoir ages in the North Atlantic surface ocean and increased atmospheric Δ 14C during the beginning of the YD event.

Sea surface temperatures and ice rafting in the Holocene North Atlantic: climate influences on northern Europe and Greenland

The oceanographic conditions in the high-latitude North Atlantic ocean during the Holocene were reconstructed through analyses of sea surface temperature (SST; alkenone unsaturation ratios) and ice rafting (mineralogy and grain size) from two sediment sequences, one recovered from the Reykjanes Ridge at 59°N and the other from the Norwegian Sea at 68°N. Comparison of our records to published ice core and terrestrial proxy-climate data sets suggests that atmospheric temperature changes over Northern Europe and Greenland were coupled to SST variability and ice rafting. The records outline four major climatic phases: (i) an early-Holocene Thermal Maximum that lasted until approximately 6.7 kyr BP, (ii) a distinctly cooler phase associated with increased ice rafting between 6.5 and 3.7 kyr BP, (iii) a transition to generally warmer, but relatively unstable climate conditions between 3.7 and 2 kyr BP and (iv) a second distinct SST decline that took place between 2 and 0.5 kyr BP. In contrast to the dominant control of Northern Hemisphere summer insolation on early-Holocene climate development (via strong seasonality), the trigger for the onset of relatively unstable climatic conditions in the North Atlantic at 3.7 kyr BP is not straightforward. However, it is possible that this change was triggered by late-Holocene winter insolation increase at high northern latitude and/or by inter-hemispheric changes in orbital forcing. The late-Holocene Neoglaciation trend, which is characteristic of numerous terrestrial archives in northern Europe, may not only be attributed to a gradual decrease in orbitally forced summer temperature, but also to increase snow precipitation at high northern latitudes during generally milder winters.