Variability of (sub)surface water masses at the Iberian Margin during the mid-Pleistocene
Final Report Abstract
The Mid-Pleistocene Revolution (MPR) at around 900 kyrs before present is the most prominent change in the climate system over the past ca. 1.5 million years. It goes along with a change in the glacial/interglacial cyclicity from an obliquity-paced 41 kyr period to an eccentricity-paced 100 kyr world, in which we are currently living. Abundant records suggest that with onset of the MPR ice ages have been getting more severe and continental ice shield growth intensified. This project aims to understand the oceanic feedbacks to these climatic changes from a subtropical point of view. In particular, it studies the variability of the Subtropical Gyre circulation in the North Atlantic. The winddriven Subtropical Gyre is a key region for understanding the Atlantic Ocean Circulation since it accumulates huge amounts warm and salt-rich waters in its interior. It thereby acts as a source for warm water advected northwards by Gulfstream and North Atlantic Current and provides the necessary high salinity waters to promote deep water formation in high latitudes. It should be expected that the general changes in the boundary conditions (especially ice volume growth) had profound impact on the Subtropical Gyre circulation by shifting wind fields and thereby the extend and location of the Subtropical Gyre. To trace such changes, temperature and salinity records from the surface and subsurface at the Iberian Margin Site U1385, located at the western boundary of the Subtropical Gyre, have been generated for the interval 700 – 1400 ka. The reconstructions are based on combined δ18O and Mg/Ca measurements on the surface dwelling foraminifera G. bulloides and deep dwelling G. inflata. Mg/Ca-derived temperatures and δ18Oseawater-estimates (approximating salinity) imply large-amplitude fluctuations in Subtropical Gyre strength before the MPR. Interestingly, the subsurface evolution does not go conform with previous surface records from the (sub)tropical North Atlantic which imply a gradual climatic deterioration, especially manifested in progressively colder glacials. Conversely, subsurface temperatures indicate a warming trend for a period prior to the MPR, a maximum cooling during the MPR but, most surprisingly, warm and stable conditions after the MPR. One explanation might be that after the MPR upper ocean stratification might have been very intense and G. inflata relocated its habitat to shallower and, thus, warmer water depths. Alternatively, it might be conceived that severe glacial cooling at the same time caused a cooling of thermocline waters as well as a deepening of the thermocline due to enhanced and southward-shifted wind fields over the gyre. Both processes would compensate each other and might lead to the low variability observed after the MPR. The latter possibility requires that thermocline deepening after the MPR had been decisively stronger than before, which might in fact be explained by stronger wind forcing due to the progressively longer and more voluminous ice-shield growth. Another factor might be less intense summer upwelling during glacials after Marine Isotope Stage 22. However, δ13C data does not show the expected shifts in nutrient-availability which argues against notable changes in upwelling intensity. Finally, some periods of unusual high thermocline temperatures and salinities go parallel with an intensification of Mediterranean Outflow Water (MOW) strength inferred from a core located in the upper branch of the MOW within the Gulf of Cadiz. Other than today, an intensification and upward-shift of the MOW might have affected thermocline properties at Site U1385 during the mid-Pleistocene. In summary, the generated records represent a novel data set linking high- and low-latitude oceanic and atmospheric variability during the MPR. To fully understand the complex interaction of different forcing mechanisms on Site U1385, other paleoenvironmental data sets (e.g. pollen and organic biomarkers), currently obtained in different laboratories, will help to better constrain the competing influence of wind forcing and surface conditions on the generated record. In addition, the integration of oceanic modeling is further envisaged and should foster our understanding of the effects of different boundary conditions on the Subtropical Gyre variability.
Publications
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(2013), Expedition 339 Summary, Proceedings IODP 339, International Ocean Drilling Program Management Int., Tokyo, Japan, 918 pp.
Stow, D.A.V., Hernández-Molina, F.J., Alvarez-Zarikian, C., Acton, G., Bahr, A., Balestra, B., Ducassou, E., Flood, R., Flores, J.-A., Furota, S., Grunert, P., Hodell, D., Jimenez-Espejo, F., Kyoung Kim, J., Krissek, L., Kuroda, J., Li, B., Llave, E., Lofi, J., Lourens, L., Miller, M., Nanayama, F., Nishida, N., Richter, C., Roque, C., Pereira, H., Sanchez Goñi, M.F., Sierro, F.J., Singh, A.D., Sloss, C., Takashimizu, Y., Tzanova, A., Voelker, A., Williams, T., and Xuan, C.
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(2014), Onset of Mediterranean Outflow into the North Atlantic, Science (344), 1244-1250
Hernández-Molina, F.J., Stow, D.A.V., Alvarez-Zarikian, C., Acton, G., Bahr, A., Balestra, B., Ducassou, E., Flood, R., Flores, J.-A., Furota, S., Grunert, P., Hodell, D., Jimenez-Espejo, F., Kyoung Kim, J., Krissek, L., Kuroda, J., Li, B., Llave, E., Lofi, J., Lourens, L., Miller, M., Nanayama, F., Nishida, N., Richter, C., Roque, C., Pereira, H., Sanchez Goñi, M.F., Sierro, F.J., Singh, A.D., Sloss, C., Takashimizu, Y., Tzanova, A., Voelker, A., Williams, T., and Xuan, C.
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Deciphering bottom current velocity and paleoclimate signals from contourite deposits in the Gulf of Cádiz during the last 140 kyr: an inorganic geochemical approach, Geochem. Geophys. Geosyst.
Bahr, A., Jimenez Espejo, F.J., Kolasinac, N., Grunert, P., Hernández-Molina, F., Röhl, U., Voelker, A., Escutia, C., Stow, D.A.V., Hodell, D., and Alvarez Zarikian, C.A.
