ALBERT

Description: ABSTRACT FINAL ID: PP23A-1831 We here present (isotope)geochemical and sedimentological data from marine sediment cores from the northern Gulf of Mexico to approximate the temporally and spatially varying terrigenous sediment contribution via the Mississippi River and the related spread of freshwater over the last glacial-interglacial cycles, with specific focus on the last ca. 42.000 years. Our study is based on cores from the DeSoto Canyon (MD02-2576 and 2575), from ~90 km southeast off the Mississippi River delta (M78-181), and from southwest of the delta (IODP 1319A). The geochemical signature of the eastern cores closely matches that of the Mississippi catchment area rather than those of the Alabama and Mobile River catchments. In particular, the siliciclastic major element potassium (K), estimated from calibrated XRF core scanning, serves as a suitable proxy for Mississippi River sediment discharge, becoming less concentrated with distance from the delta. The K variability suggests enhanced glacial phase terrigenous influx triggered by strengthened fluvial runoff and changing fluvial and ice sheet dynamics. Mississippi River influx was at a maximum during glacial MIS 2/3, late MIS 8 and MIS 10, reflected by sedimentation rates being 4 to 5 times higher than in the Holocene. Late glacial to deglacial fluvial sediment supply, however, decreased abruptly at ca. 20 ka at our easternmost core location (MD02-2576), and ca. 2 kyr later at our core location closest to the Mississippi Delta, implying a gradual westward shift of the Mississippi outflow. Due to synchronous changes in sea-surface temperatures, we hypothesize an increasing impact of the northward extending Loop Current on the Mississippi outflow pattern. Combined stable oxygen isotope and element ratios from shallow and deep-dwelling as well as benthic foraminifers allow to approximate paleosalinity, and hence to follow the dispersal of freshwater across the Gulf of Mexico. According to our data, Mississippi freshwater discharge events appeared during the last glacial and during Termination II, but were mostly confound to the southwest of the Mississippi River delta. The prominent discharge event during Heinrich 1 and the Boelling/Alleroed warm period is also observed at core location M78-181, but not further to the east. Notably, sediment supply during this megadischarge event is insignificant compared to full glacial conditions, suggesting that signals of freshwater and sediment supply became decoupled. Holocene changes of Mississippi discharge are closely related to the sea-surface temperature and salinity development in the northern Gulf, most likely amplified by the migration of the Innertropical Convergence Zone, related dislocations of the Hadley Cell, and changes in climatic zones.

Description: Subtropical Gyres are an important constituent of the ocean–atmosphere system due to their capacity to store vast amounts of warm and saline waters. Here we decipher the sensitivity of the (sub)surface North Atlantic Subtropical Gyre with respect to orbital and millennial scale climate variability between ~ 140 and 70 ka, Marine Isotope Stage (MIS) 5. Using (isotope) geochemical proxy data from surface and thermocline dwelling foraminifers from Blake Ridge off the west coast of North America (ODP Site 1058) we show that the oceanographic development at subsurface (thermocline) level is substantially different from the surface ocean. Most notably, surface temperatures and salinities peak during the penultimate deglaciation (Termination II) and early MIS 5e, implying that subtropical surface ocean heat and salt accumulation might have resulted from a sluggish northward heat transport. In contrast, maximum thermocline temperatures are reached during late MIS 5e when surface temperatures are already declining. We argue that the subsurface warming originated from intensified Ekman downwelling in the Subtropical Gyre due to enhanced wind stress. During MIS 5a-d a tight interplay of the subtropical upper ocean hydrography to high latitude millennial-scale cold events can be observed. At Blake Ridge, the most pronounced of these high latitude cold events are related to surface warming and salt accumulation in the (sub)surface. Similar to Termination II, heat accumulated in the Subtropical Gyre probably due to a reduced Atlantic Meridional Overturning Circulation. Additionally, a southward shift and intensification of the subtropical wind belts lead to a decrease of on-site precipitation and enhanced evaporation, coupled to intensified gyre circulation. Subsequently, the northward advection of this warm and saline water likely contributed to the fast resumption of the overturning circulation at the end of these high latitude cold events.

Description: Introduction In a comment on our recent paper (Karas et al., 2011a) Dickens and Backman argue that our interpretation of planktonic δ18O and Mg/Ca data from the southwestern Pacific DSDP Site 590B for the time period 5.0 to 4.3 Ma is difficult to reconcile with previous notions on early Pliocene upwelling phenomena and subsequent surface warming at the same site. Our study for the first time presented Pliocene Mg/Ca-derived temperatures and δ18Oivc-seawater values approximating changes in salinities at Site 590B for both surface and subsurface levels. It is hitherto the only study, which reconstructs absolute temperatures for the Pliocene with a substantially higher time resolution (~ 9 kyrs) than previous geochemical and faunal records from Site 590 (e.g., ~ 250 kyrs in Elmstrom and Kennett, 1986; [Grant and Dickens, 2002] and [Kennett and von der Borch, 1986]). Here, we attempt to address the issues raised by Dickens and Backman and synthesize the so far diverging interpretations in order to support the reliability of our study.

Description: The impacts of the constrictions of the Indonesian Gateway and the Central American Seaway on ocean circulation are among the keys to understand Pliocene climate evolution, including the intensification of the Northern Hemisphere Glaciation between 3.5 and 2.5 Ma. Plate tectonic reconstructions show that the main reorganization of one such seaway, the Indonesian Gateway, occurred between 4 and 3 Myr ago. Model simulations have suggested that this tectonic reorganization triggered far-reaching effects on ocean circulation and climate, including a switch in the source of waters feeding the Indonesian Throughflow into the Indian Ocean. This PHD thesis aims to elucidade the climatic and oceanographic changes related to the constriciton of the Indonesian Gateway. It presents combined d18O and Mg/Ca ratios of planktonic foraminifera (marine protozoa) from surface and subsurface levels to reconstruct the thermal structure and changes in salinities at four following sensitive core sites in the Indian and Pacific Oceans from ~6 to 2 Myr ago: DSDP Site 214 in the tropical east Indian Ocean, ODP Site 709C in the west tropical Indian Ocean, ODP 763A in the subtropical east Indian Ocean under the influence of the Leeuwin Current, and DSDP Site 590B in the southwest Pacific Ocean at the Tasman Front: In the outflow region of the Indonesian Throughflow (DSDP Site 214), sea surface conditions remained relatively stable throughout the mentioned Pliocene interval, while subsurface waters (300-450 m water depth) freshened and cooled by about 4°C between 3.5 and 2.95 Myr ago. After 2.95 Ma, constantly low subsurface temperatures and fresher conditions suggested a prevailing throughflow of North Pacific source waters through the Indonesian Gateway. These findings supported the hypothesis of Cane and Molnar (2001) that the constriction of the Indonesian Gateway (4-3 Ma) led to a major reorganization in the Indonesian Throughflow. The cooling and shoaling of the thermocline in the tropical Indian Ocean might have contributed to cooling in various (sub)tropical upwelling regions. At Site 763, surface temperatures cooled by ~2°C compared to tropical Indian Ocean sites 214 and 709C during the mid-Pliocene, pointing to a Leeuwin Current, which weakened since ~3.3 Myr ago in line with the hydrographic changes in the Indonesian Throughflow region. Most likely, a reduced surface Indonesian Throughflow led to a diminished poleward heat transport resulting in a weakened Leeuwin Current and a cooling of the Benguela upwelling system. Thereby, by cooling the southern Indian and Atlantic Oceans this mechanism amplified the mid-Pliocene global development towards increased meridional temperature gradients. The Tasman Front Site 590B is influenced by both, the Central American Seaway and the Indonesian Gateway. Gradual cooling of ~2°C, and freshening of the sea surface during ~4.6-4 Ma was related to the closing of the Central American Seaway, which reached a critical threshold during this time and presumably cooled the southwest Pacific through heat piracy by the Northern Hemisphere. After ~3.5 Ma, the ongoing restriction of the Indonesian Gateway might have amplified the southward heading East Australian Current, allowing still warm sea surface temperatures at Site 590B when the global climate gradually cooled. In contrast, the cooling and freshening of the subsurface level in line with a marked increase in the sandfraction points to a fostered northward circulation of Subantartic Mode- and Antartic Intermediate waters, possibly a first step towards the present Antarctic Frontal System.