A decadal resolution time series of sea surface temperature (SST) spanning the last two millennia is reconstructed by combining a proxy record from a new sediment sequence with previously published ...data from core MD99–2275, north of Iceland. The alkenone based SST reconstruction is validated with historic observational data and compared to a new similar temporal resolution reconstruction obtained from sediment core RAPiD21–3K, in the subpolar North Atlantic. The two SST paleorecords show consistent multidecadal scale coolings throughout the interval and similar expressions during the contrasted climatic periods ‘colloquially known’ as the Medieval Climatic Anomaly (MCA) and Little Ice Age (LIA). In order to further understand the temporal and spatial SST variations and investigate the influence of natural forcings on the observed SST changes during the last millennium, we compare our time series to simulations using the Institut Pierre‐Simon Laplace IPSLCM4‐v2 climate model. This comparison highlights the potential importance of volcanism as a natural forcing driving coherent abrupt cooling events captured in the subpolar North Atlantic records.
Key Points
Comparative analysis of marine paleo proxy data to simulation of the last 1000 years
The impact of volcanism on ocean variability
The ice-loaded Labrador Current (LC) is an important component of the western North Atlantic circulation that influences the position and strength of the northern limb of the North Atlantic Current ...(NAC). This flow of cold and fresh Polar Waters originating from the Arctic has a marked impact on the North Atlantic climate, yet little is known about its variability beyond the instrumental period. In this study, we present the first sub-decadal alkenone-based 2000-year long sea-surface temperature (SST) records from the western Labrador Sea, a climatically crucial region at the boundary between the LC and the NAC. Our results show a clear link between the LC strength and the Northern Annular Mode (NAM), with a stronger NAM and a more vigorous LC during the Medieval Climate Anomaly (MCA). This suggests enhanced LC activity upon future global warming with implications for the Atlantic meridional overturning circulation (AMOC).
•First subdecadal-scale SSTs (alkenones) in the Labrador Sea over the last 2k.•Role of NAM index on SSTs (direct atmospheric forcing and Arctic waters gateways).•NAM/proxy SSTs correlation agrees with modern correlation map of NAO/AO vs SSTs.•Role of NAM on LIA and MCA in the Labrador Sea/Northern Gulf Stream boundary.•Implications of our findings future climate in Northeastern America.
•The freshwater content (FWC) appears to be a crucial factor of the phytoplankton response to summer sea ice retreat, by acting on the nutrient reservoir depth.•The strong freshening observed in the ...Canada Basin had a negative impact on primary producers.•Biomasses accumulation and relatively high primary production were observed across the offshore marginal ice zone.•The Chukchi shelf, with the lower FWC, was the most productive area of the cruise with biomasses and primary production values in the range of previous studies.
Since the 1990s, drastic melting of sea ice and continental ice in the Arctic region, triggered by global warming, has caused substantial freshening of the Arctic Ocean. While several studies attempted to quantify the magnitude of this freshening, its consequences on primary producers remain poorly documented. In this study, we evaluate the impact of the freshwater content (FWC) of the upper Arctic Ocean on phytoplankton across the Pacific sector, from the Bering Strait (65°N) to the North Pole (86°N), during summer 2008. We performed statistical analyses on the physical, biogeochemical and biological data acquired during the CHINARE 2008 cruise to investigate the effect of sea-ice melting on the Arctic phytoplankton. We found that the strong freshening observed in the Canada Basin had a negative impact on primary producers as a result of the deepening of the nitracline and the establishment of a subsurface chlorophyll maximum (SCM). In contrast, regions with lower freshening, such as the Chukchi shelf and the marginal ice zone (MIZ) over the Chukchi Borderland, exhibited a shallower nitracline sustaining relatively high primary production and biomass. Our results imply that the predicted increase freshening in future years will likely cause the Arctic deep basin to become more oligotrophic because of weaker surface nutrient renewal from the subsurface ocean, despite higher light penetration.
Despite a large number of studies, the long-term and millennial to centennial-scale climatic variability in the Mediterranean region during the last deglaciation and the Holocene is still debated, ...including in the southern Central Mediterranean. In this paper, we present a new marine pollen sequence (core MD04-2797CQ) from the Siculo-Tunisian Strait documenting the regional vegetation and climatic changes in the southern Central Mediterranean during the last deglaciation and the Holocene. The MD04-2797CQ marine pollen sequence shows that semi-desert plants dominated the vegetal cover in the southern Central Mediterranean between 18.2 and 12.3 ka cal BP, indicating prevailing dry conditions during the deglaciation, even during the Greenland Interstadial (GI)-1. Across the transition Greenland Stadial (GS)-1 - Holocene, Asteraceae-Poaceae steppe became dominant till 10.1 ka cal BP. This record underlines with no chronological ambiguity that even though temperatures increased, deficiency in moisture availability persisted into the early Holocene. Temperate trees and shrubs with heath underbrush or maquis expanded between 10.1 and 6.6 ka, corresponding to Sapropel 1 (S1) interval, while Mediterranean plants only developed from 6.6 ka onwards. These changes in vegetal cover show that the regional climate in southern Central Mediterranean was wetter during S1 and became drier during the mid- to late Holocene. Wetter conditions during S1 were likely due to increased winter precipitation while summers remained dry. We suggest, in agreement with published modeling experiments, that the early Holocene increased melting of the Laurentide Ice Sheet in conjunction with weak winter insolation played a major role in the development of winter precipitation maxima in the Mediterranean region in controlling the strength and position of the North Atlantic storm track. Finally, our data provide evidence for centennial-scale vegetation and climatic changes in the southern Central Mediterranean. During the wet early Holocene, alkenone-derived cooling episodes are synchronous with herbaceous composition changes that indicate muted changes in precipitation. In contrast, enhanced aridity episodes, as detected by strong reduction in trees and shrubs, are recorded during the mid- to late Holocene. We show that the impact of the Holocene cooling events on the Mediterranean hydroclimate depend on baseline climate states, i.e. insolation and ice sheet extent, shaping the response of the mid-latitude atmospheric circulation.
Radiocarbon dates and marine tephra suggest that the upper 10 m of core MD99-2274 off North Iceland extends from ∼0 to ∼65 ka BP. A multi-proxy sediment and biomarker study at a ∼0.5 ky resolution is ...used to derive a paleoclimate scenario for this area of the southwestern Nordic Seas, which during the Holocene had intermittent excursions of icebergs and a seasonal cover of drifting sea ice across the site. The sortable silt mean size (S̅S̅) suggests a bottom current (1000 m depth) flow speed maximum to minimum range of ∼8 cm/s during Marine Isotope Stages 2–3, but the data are unreliable for the Holocene. Slow-down in flow speeds may be associated with massive ice and water discharges linked to the Hudson Strait ice stream (H-events) and to melt of icebergs from Greenland in the Nordic seas where convection would have been suppressed. Five pulses of sediment with a distinct felsic component are associated with iceberg transport from E/NE Greenland. Sea ice, open water and sea surface temperature (SST) biomarker proxies (i.e. IP25, HBI III, brassicasterol and alkenones) all point towards near-perennial sea ice cover during MIS 3 and 2, rather than seasonal sea ice or open water conditions. Indeed, our biomarker and sediment data require that the seas north of Iceland experienced a nearly continuous cover of sea ice, together with icebergs calved from ice stream termini, which drifted southward. The cross-correlation of the quartz % records between MD99-2274 and the well-dated core PS2644 in Blosseville Basin indicates significant coherence in the records at a multi-millennial (∼8 ky) timescale. A transition to open ocean conditions is evident from the early Holocene onwards, albeit with the occurrence of some drift ice and icebergs.
•Multi-proxy biogeochemical, grain-size, and mineral analysis.•MIS 2 and 3 record of sea ice cover and ice-rafting in the Iceland Sea.•HBI, sterol and alkenone biomarkers point to extensive sea ice during MIS 3 and MIS 2.•Episodes of quartz input linked to a NE source (s).•Bottom currents at 1000 m depth combine THC Nordic outflow and saline density flow.
Paleohydrological changes in the southern South China Sea (SCS) combined with clay mineralogy have been investigated along core MD01-2393 recovered off the Mekong River mouth in order to assess the ...impact of sea level and East Asian monsoon rainfall intensity on erosion and weathering during the last 25,000
yr. SSTs and
δ
18O values determined on
Globigerinoides ruber were used to estimate past changes of local seawater oxygen isotope (
δ
18O
w). The close position of the studied core to the Mekong River mouth at sea level lowstand likely played a role in the
δ
18O
w fluctuations resulting from changes of the monsoon rainfall and runoff into the Mekong River basin. The smectite/(illite
+
chlorite) and kaolinite/(illite
+
chlorite) ratios combined with the illite chemistry index during the Holocene show higher chemical weathering of detrital material originating mainly from the lower reach of the Mekong River. At shorter time scales, periods of strong monsoon rainfall are associated with an intensification of erosion of the Mekong River lowland favoured by the development of incised-valley systems inducing higher inputs of detrital material from the lower relative to the upper reach of the Mekong River. Our findings imply a rapid response of erosion processes of the Mekong River basin to the monsoon rainfall intensity changes.
In this study, we use IP25 and alkenone biomarker proxies to document the subdecadal variations of sea ice and sea surface temperature in the subpolar North Atlantic induced by the decadally paced ...explosive tropical volcanic eruptions of the second half of the thirteenth century. The short‐ and long‐term evolutions of both variables were investigated by cross analysis with a simulation of the IPSL‐CM5A LR model. Our results show short‐term ocean cooling and sea ice expansion in response to each volcanic eruption. They also highlight that the long response time of the ocean leads to cumulative surface cooling and subsurface heat buildup due to sea ice capping. As volcanic forcing relaxes, the surface ocean rapidly warms, likely amplified by subsurface heat, and remains almost ice free for several decades.
Key Points
ocean response to volcanism combining proxy and model data
Impact of closely spaced eruptions on SSTs and sea ice
long‐term response of high latitude ocean to volcanism
Past and present environmental conditions over the Holocene along the Algerian coast involve complex atmosphere-hydrosphere-biosphere interactions and anthropogenic activities on adjacent watersheds. ...Atlantic Ocean surface waters entering the western Mediterranean Sea at the Gibraltar Strait create the Algerian Current, which flows along the North African coast in a succession of strong and large-scale eddies. Deep-water upwelling plumes are other recurrent hydrological features of the Algerian margin affecting regional environmental features. However, vegetation and paleohydrological changes that have occurred over the Holocene have not yet been described. To bridge this gap, a suite of paleoclimate proxies was analysed in marine core MD04–2801 (2067 m water depth) at a secular-scale resolution over the last 14 kyrs BP. Terrestrial (pollen grains) and marine (dinoflagellate cysts or dinocysts) palynological assemblages, as well as sedimentological (grain-size analysis and XRD-based quantitative analysis of clay minerals) and biomarkers (alkenones and n-alkanes), were determined to explore the links between past sea surface hydrological conditions and regional environmental changes on nearby watersheds.
The over-representation of heterotrophic dinocyst taxa (Brigantedinium spp.) indicates strong planktonic productivity in the study area. Results shows that the links between dryness on land and surface hydrological conditions are expressed by: (i) recurrent upwelling cells during the relatively dry climate conditions of the Younger Dryas (12.7 to 11.7 ka BP), the Early Holocene (11.7 to 8.2 ka BP) and from 6 ka BP onwards, (ii) enhanced fluvial discharges between 8.2 and 6 ka BP during the African Humid Period concomitant with the colonization of coastal lands by Mediterranean forest. Middle to Late Holocene transition around 4.2 ka BP characterizes by the intense event reffered to here as the Algerian Mega Drought (4.3 to 3.9 ka BP).
•Land-sea multiproxy approach on the Algerian Margin over the last 14 kyrs.•Productivity regimes closely associated with the vigour of the Algerian Current.•Settlement of modern production conditions since 3 ka BP.•First record of the Algerian Mega Drought event between 4.3 and 3.9 ka BP.•NPP fingerprint in marine sediments related to erosion and river runoff.
The Nd isotopic composition of the aragonite skeleton of fossil deep-sea corals (
Lophelia pertusa,
Madrepora oculata and
Desmophyllum dianthus) located in the northeastern Atlantic at water depths ...between 635 and 1300
m was investigated to reconstruct changes in the Atlantic mid-depth gyre circulation during the past millennium. The coral εNd values varied systematically from −
11.8 to −
14.4 during the past 1500
years, reflecting variations in seawater εNd and thus water mass provenance. Low εNd values (εNd
=
−
14) occurred during the warm Medieval Climatic Anomaly (MCA) (between 1000
AD and 1250
AD) and during the most recent period (1950
AD to 2000
AD), interrupted by a period of significantly higher εNd values (~−12.5) during the Little Ice Age (LIA) (between 1350
AD and 1850
AD). One long-lived branching coral even recorded an abrupt systematic rise from low to high εNd values around 1250
AD over the course of its 10-year growth period.
These variations are interpreted to result from variable contributions of the subpolar and subtropical Atlantic intermediate water masses, which today are characterized by εNd values of −
15 and ~−11, respectively. The low εNd values observed during the warm MCA and during recent times imply a strong eastward extension of the mid-depth subpolar gyre (SPG) induced by a dominant positive phase of the North Atlantic oscillation (NAO). During the LIA, water from the subtropical gyre (STG) and potentially from the Mediterranean Sea Water (MSW) propagated further northward, as indicated by the higher coral εNd values. This pattern suggests a negative mean state of the NAO during the LIA, with weaker and more southerly located Westerlies and a westward contraction of the SPG. Variations in the contributions of the two gyres imply changes in the heat and salt budgets at intermediate depths during the past millennia that may have contributed to changes in the properties of North Atlantic inflow into the Nordic Seas and thus deep-water formation.
► εNd of U–Th dated deep-sea corals from Rockall Trough have been investigated. ► Corals εNd indicate strong hydrological changes during the last 1500
yrs. ► This εNd variability indicates major changes in mid-depth gyre circulation (STG-SPG). ► The warm MCA is characterized by a strong eastward extension of the mid-depth SPG. ► This mid-depth hydrological re-organization is attributed to a change in the NAO.
Dissolved and particulate PAHs were quantified throughout the Seine River and its estuary. Samples were collected in October 1993, covering a salinity gradient of 0.2‰ to 34.8‰. Two mooring stations ...were occupied at the river mouth to ascertain the influence of tidal cycles on the dispersal of riverborne PAHs. Total particulate PAH concentrations ranged from 2 to 687 ng l
−1 (or 1 to 14 μg g
−1). Concentrations were correlated to the suspended matter load and distributions could be explained by estuarine mixing. PAH levels decreased from ebb to flood tides. PAH concentrations in the dissolved phase (4 to 36 ng l
−1) were, in general, an order of magnitude lower than in the particulate phase. The partition coefficient of individual PAHs (K
oc) were comparable to those previously reported for the Rhône River suspensions.
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