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- Pronounced northward shift of the westerlies during MIS 17 leading to the strong 100-kyr ice age cyclesPublication . Goni, Maria Fernanda Sanchez; Ferretti, Patrizia; Polanco-Martinez, Josue M.; Rodrigues, Teresa; Alonso-Garcia, Montserrat; Javier Rodriguez-Tovar, Francisco; Dorador, Javier; Desprat, StephanieThe MIS 17 interglacial, similar to 715-675 ka, marks the end of the Mid-Pleistocene Transition as intensified, long and asymmetrical 100-kyr ice age cycles became eminently established. Increasing arrival of moisture to the Northern Hemisphere high latitudes, resulting from the northwestward migration of the Subpolar Front and the intensification of the Norwegian Greenland Seas (NGS) convection, has been put forward to explain the emergence of this quasi-periodic 100-kyr cycle. However, testing this hypothesis is problematic with the available North Atlantic precipitation data. Here we present new pollen-based quantitative seasonal climate reconstructions from the southwestern Iberian margin that track changes in the position and intensity of the westerlies. Our data compared to changes in North Atlantic deep and surface water conditions show that MIS 17 interglacial was marked by three major changes in the direction and strength of the westerlies tightly linked to oceanographic changes. In particular, we report here for the first time a drastic two-steps northward shift of the westerlies centered at similar to 693 ka that ended up with the sustained precipitation over southern European. This atmospheric reorganization was associated with northwestward migration of the Subpolar Front, strengthening of the NGS deep water formation and cooling of the western North Atlantic region. This finding points to the substantial arrival of moisture to the Northern Hemisphere high latitudes at the time of the decrease in summer energy and insolation contributing to the establishment of strong 100-kyr cycles. (C) 2019 Elsevier B.V. All rights reserved.
- Sea-level and monsoonal control on the Maldives carbonate platform (Indian Ocean) over the last 1.3 million yearsPublication . Alonso-Garcia, Montserrat; Reolid, Jesus; Jimenez-Espejo, Francisco J.; Bialik, Or M.; Alvarez Zarikian, Carlos A.; Laya, Juan Carlos; Carrasquiera, Igor; Jovane, Luigi; Reijmer, John J. G.; Eberli, Gregor P.; Betzler, ChristianThe Maldives Archipelago (Indian Ocean), composed of two rows of atolls that enclose an inner sea, offers an excellent study site to explore the forcings of carbonate production at platforms. Glacial-interglacial sea-level changes have been claimed to be the main factor controlling the carbonate platform factories; however, climatic factors may also have an impact. In this work we used geochemical compositional records, obtained by X-ray fluorescence (XRF) core-scanning from the International Ocean Discovery Program (IODP) Site U1467 in the Maldives Inner Sea, to analyze the orbitally driven fluctuations on the carbonate production and export from the neritic environment into the Maldives Inner Sea over the last 1.3 million years.High Sr aragonite-rich carbonates (HSAC) from neritic settings were deposited in the Maldives Inner Sea during sea-level highstand intervals, increasing the Sr / Ca values. In contrast, low Sr / Ca values are observed coincident with sea-level lowstand periods, suggesting that large areas of the atolls were exposed or unable to grow, and therefore, there was a demise in the carbonate production and sediment export to the Maldives Inner Sea. However, comparison of the Sr / Ca values and the sea-level reconstructions for different interglacial periods before and after the mid-Brunhes event (MBE, similar to 430 ka ) indicates that sea level is not the only factor controlling the production of HSAC during sea-level highstands. The study of monsoon and primary productivity proxies ( Fe -normalized, Fe / K , and Br -normalized records) from the same site suggests that the intensity of the summer monsoon and the Indian Ocean dipole probably modulated the carbonate production at the atolls. Moreover, Marine Isotope Stage 11 stands out as a period with high sea level and extraordinary carbonate production in the Maldives platform. This outstanding carbonate production in the Maldives atolls (and in other low-latitude carbonate platforms) probably contributed to the mid-Brunhes dissolution event through a strong shelf-to-basin fractionation of carbonate deposition.
- Freshening of the labrador sea as a trigger for little ice age developmentPublication . Alonso-Garcia, Montserrat; Kleiven, Helga (Kikki) F.; McManus, Jerry F.; Moffa-Sanchez, Paola; Broecker, Wallace S.; Flower, Benjamin P.Arctic freshwater discharges to the Labrador Sea from melting glaciers and sea ice can have a large impact on ocean circulation dynamics in the North Atlantic, modifying climate and deep water formation in this region. In this study, we present for the first time a high resolution record of ice rafting in the Labrador Sea over the last millennium to assess the effects of freshwater discharges in this region on ocean circulation and climate. The occurrence of ice-rafted debris (IRD) in the Labrador Sea was studied using sediments from Site GS06-144-03 (57.29 degrees N, 48.37 degrees W; 3432 m water depth). IRD from the fraction 63-150 mu m shows particularly high concentrations during the intervals similar to AD 1000-1100, similar to 1150-1250, similar to 1400-1450, similar to 1650-1700 and similar to 1750-1800. The first two intervals occurred during the Medieval Climate Anomaly (MCA), whereas the others took place within the Little Ice Age (LIA). Mineralogical identification indicates that the main IRD source during the MCA was SE Greenland. In contrast, the concentration and relative abundance of hematite-stained grains reflects an increase in the contribution of Arctic ice during the LIA. The comparison of our Labrador Sea IRD records with other climate proxies from the subpolar North Atlantic allowed us to propose a sequence of processes that led to the cooling that occurred during the LIA, particularly in the Northern Hemisphere. This study reveals that the warm cli- mate of the MCA may have enhanced iceberg calving along the SE Greenland coast and, as a result, freshened the subpolar gyre (SPG). Consequently, SPG circulation switched to a weaker mode and reduced convection in the Labrador Sea, decreasing its contribution to the North Atlantic deep water formation and, thus, reducing the amount of heat transported to high latitudes. This situation of weak SPG circulation may have made the North Atlantic climate more unstable, inducing a state in which external forcings (e.g. reduced solar irradiance and volcanic eruptions) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. This analysis indicates that a freshening of the SPG may play a crucial role in the development of cold events during the Holocene, which may be of key importance for predictions about future climate.