The Tagus Prodelta (W Portugal) and the Muros Ría (NW Spain) are areas of high deposition rates registering high-resolution palaeoclimatic records for western Iberia. We compare the climatic conditions of the two areas over the last two millennia based on proxies of temperature (sea surface temperatures and oxygen isotopes), continental input (grain size, iron and magnetic susceptibility) and productivity (inorganic and organic carbon, carbon isotopes, benthic foraminifera and diatoms). Biogeochemical changes in the Tagus Prodelta reflect widely recognized North Atlantic climatic periods encompassing the Roman Period (AD 0-350), the Dark Ages (AD 400-700), the ‘Mediaeval Warm Period’ (MWP; AD 800-1200) and the ‘Little Ice Age’ (LIA; AD 1300-1750). The atmospheric North Atlantic Oscillation (NAO) drives the Tagus Prodelta multidecadal, long-term variability in precipitation-river input during cold periods (negative NAO) and marine upwelling during warmer periods (positive NAO), a scheme that is reversed in the Galician region. The Muros Ría shows only local hydrodynamics until AD 1150, including a ‘suboxic’ event in the inner Ría around AD 500-700. Since AD 1150 Atlantic warm upwelled waters have ventilated the outer Ría but only reach the inner Ría at AD 1750. The twentieth-century records are also interpreted as a reflex of the inverse NAO mode in both areas, resulting in amplification of the LIA biogeochemical water conditions. Centennial-scale solar activity appears to be another important forcing mechanism (or the only one, if solar activity drives the NAO and ‘Bond-cycles’) behind changes in the hydrography of the Tagus Prodelta, and primary production, bottom ventilation and organic carbon degradation in the Muros Ría.
Millennial‐scale climate events in the North Pacific are thought to be related to changes in the circulation of North Pacific Intermediate Water, which may have formed in the Bering Sea in the past. To advance our understanding of the mechanisms that underlie millennial‐scale events, Bering Sea sediment cores from the Integrated Ocean Drilling Program site U1340 were used to construct high‐resolution, multiproxy climate records of the last 90,000 years. Sediment density records show millennial‐scale events resembling Dansgaard‐Oeschger events, several of which are laminated. Interstadials were characterized by 3–5 °C warming, increased productivity driven by upwelling, and reduced benthic oxygenation. Bering Sea intermediate water also changed over longer timescales; our records show the presence of intermediate water with lower salinity and higher oxygen content than modern beginning around 60 ka and persisting until the beginning of the deglaciation. The Bølling‐Allerød was characterized by high productivity, laminated sediments, and strong denitrification signature. Our data support the idea that productivity‐derived changes in oxygenation at intermediate water source regions may have contributed to the intensification of the North Pacific–wide oxygen minima during the Bølling‐Allerød.
Fossil diatoms are the principal component of Bering Sea sediments and reflect the paleoceanographic history of the region. Diatom accumulation rates and relative abundances at International Ocean Discovery Program (IODP) Site U1340A are presented. Overall, the total diatom productivity record from 4.9 Ma to the present day reveals a fourfold reduction at circa 4.2 Ma from~45 × 10 7 down to 11 × 10 7 valves/g (wet sediment), signifying a major shift in the upwelling and/or nutrient regime, coinciding with the end of the late Miocene-early Pliocene bloom identified in the eastern equatorial Pacific and California margin. Further abrupt shifts in the diatom assemblage occur at (1) 2.78-2.55 Ma, (2) 2.0-1.8 Ma, and (3) 1.0-0.88 Ma. (1) At 2.78-2.55 Ma, the appearance of sea ice-related species marks the regional cooling associated with the expansion of Northern Hemisphere ice sheets, subsequent development of stratified, nutrient-depleted waters, and increased influence of Western Basin Water masses (most likely due to the suppressed inflow of the Alaskan Stream). (2) Rapid cooling between 2.0 and 1.8 Ma indicates increased sea ice duration and/or frequency. This, coupled with low sea level stands caused prolonged closure of the Aleutian Passes, coupled with further increased Western Basin Water inflow. (3) The shift to 100 ka glacial/interglacial cycles at the middle-Pleistocene transition (1.0-0.88 Ma) marked an increase in upwelling-related species, indicating enhanced surface water mixing. These records confirm that the development and changing dynamics of sea ice in the Bering Sea played a major role in sub-Arctic Ocean circulation and is an integral component of global climate change.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.