Simon Jung

surface and deep water changes in the subpolar North Atlantic during Termination II and the Last Interglaciation

To reconstruct the history of water mass exchange between the NE Atlantic and the Nordic seas, sediment cores from ∼2 km water depth were studied across Termination II (TII) and through the last interglaciation (MIS5e). During early TII the sudden appearance of the low-latitude planktonic foraminifera Beella megastoma is noted in both regions along with a steep decrease in benthic foraminiferal δ18O. Since other proxies indicate that surface waters were cold and stratified because of meltwater, conditions which prevented near-surface thermohaline circulation and vertical convection in the Nordic seas, water mass exchange between the two areas occurred at the subsurface. During later TII, surface conditions changed, and this subsurface circulation style was eventually replaced by vertical convection. In the Nordic seas, B. megastoma vanished from the record together with ice-rafted debris (IRD) at the end of TII, while subpolar foraminiferal abundance rose. Peak interglacial conditions with intensive vertical convection now fully developed, generating a bottom water temperature gradient of ∼4°C between the two areas. However, surface water temperatures deteriorated in the Nordic seas already notably before IRD recurred, and δ18O increased at the end of MIS5e.

Western Arabian Sea SST during the penultimate interglacial: A comparison of U37K′ and Mg/Ca paleothermometry

Millennial-scale records of planktonic foraminiferal Mg/Ca, bulk sediment U37 K′, and planktonic foraminiferal δ 18O are presented across the last two deglaciations in sediment core NIOP929 from the Arabian Sea. Mg/Ca-derived temperature variability during the penultimate and last deglacial periods falls within the range of modern day Arabian Sea temperatures, which are influenced by monsoon-driven upwelling. The U37 K′-derived temperatures in MIS 5e are similar to modern intermonsoon values and are on average 3.5°C higher than the Mg/Ca temperatures in the same period. MIS 5e U37 K′ and Mg/Ca temperatures are 1.5°C warmer than during the Holocene, while the U37 K′-Mg/Ca temperature difference was about twice as large during MIS 5e. This is surprising as, nowadays, both proxy carriers have a very similar seasonal and depth distribution. Partial explanations for the MIS 5e U37 K′-Mg/Ca temperature offset include carbonate dissolution, the change in dominant alkenone-producing species, and possibly lateral advection of alkenone-bearing material and a change in seasonal or depth distribution of proxy carriers. Our findings suggest that (1) Mg/Ca of G. ruber documents seawater temperature in the same way during both studied deglaciations as in the present, with respect to, e.g., season and depth, and (2) U37 K′-based temperatures from MIS 5 (or older) represent neither upwelling SST nor annual average SST (as it does in the present and the Holocene) but a higher temperature, despite alkenone production mainly occurring in the upwelling season. Further we report that at the onset of the deglacial warming, the Mg/Ca record leads the U37 K′ record by 4 ka, of which a maximum of 2 ka may be explained by postdepositional processes. Deglacial warming in both temperature records leads the deglacial decrease in the δ 18O profile, and Mg/Ca-based temperature returns to lower values before δ 18O has reached minimum interglacial values. This indicates a substantial lead in Arabian Sea warming relative to global ice melting.

Centennial-millennial-scale monsoon variations off Somalia over the last 35 ka

Abstract We present a multi-proxy study of sediment Core 905 from the Arabian Sea offshore Somalia to assess the validity of a number of proxies for productivity, temperature and wind strength, to reconstruct the monsoon history in the western Arabian Sea. The present-day seasonal variation in productivity in the modern Arabian Sea off Somalia reflects the change from the high-productivity SW monsoon to the low-productivity NE monsoon seasons. Annual productivity is therefore largely controlled by SW monsoon driven upwelling. The geochemical records of Core 905 document millennial-scale variations, for example, in Ba/Al and Corg content. The Younger Dryas and the time equivalent period to Heinrich event 1 show low annual productivity whereas the early Holocene and Bølling-Allerød periods are characterized by high productivity. The upwelling-productivity peaked during Early Holocene time and was followed by a decrease toward the modern values. The total flux of planktic foraminifera and the concentration of the planktic foraminifera G. bulloides are not always controlled by the total productivity. Variations in calcite dissolution, the advection of expatriate fauna or a seasonal decoupling of primary and secondary production appear to hamper straightforward interpretations of those foraminifera records. We conclude that at significantly changed climatic boundary conditions compared with the present day, bulk-sediment-related proxies of productivity more consistently record the local upwelling history than foraminifer-based productivity proxies.

Local and regional trends in Plio‐Pleistocene δ18O records from benthic foraminifera

We present new orbital-resolution Pliocene-Pleistocene benthic stable oxygen isotope (δ18Ob) records from Ocean Drilling Program Sites 1264 and 1267, from Walvis Ridge in the Southeast Atlantic. We compare long-term (>250 kyr) interbasin δ18Ob-gradients between Pacific and North Atlantic regional stacks, as well as intra and interbasin gradients from the perspective of Walvis Ridge. The δ18Ob values from Sites 1264 and 1267 are almost always higher than deep North Atlantic and Pacific sites, with large gradients (>0.5‰) emerging abruptly at ∼2.4 Ma and persisting until ∼1.3 Ma. From this, we infer the presence of a new water mass, which resulted from the influence of dense, 18O-enriched Nordic sea overflow waters via the abyssal East Atlantic. Meanwhile, long-term average δ18Ob values in the North Atlantic appear to have remained within 0–0.25‰ lower than in the Pacific. However, the magnitude of this difference is sensitive to the inclusion of records from the equatorial West Atlantic. These results, together with constraints based on temperature, salinity, and density, suggest an influence of the seawater δ18O (δ18OSW) versus salinity relationship of source waters on δ18Ob values within the Atlantic. In particular, the abrupt emergence at ∼2.4 Ma of higher δ18Ob values at Sites 1264 and 1267, relative to North Atlantic records, appears to require a low-latitude surface water δ18OSW signal. This implies a connection between northward heat transport and deep water export into the abyssal East Atlantic. Hence, our results have implications for the interpretation of δ18Ob records and highlight the potential for δ18Ob to constrain deep Atlantic water mass sources and pathways during the Plio-Pleistocene.

Geochemical imprints of genotypic variants of Globigerina bulloides in the Arabian Sea

Planktonic foraminifera record oceanic conditions in their shell geochemistry. Many palaeoenvironmental studies have used fossil planktonic foraminifera to constrain past seawater properties by defining species based on their shell morphology. Recent genetic studies, however, have identified ecologically distinct genotypes within traditionally recognized morphospecies, signaling potential repercussions for palaeoclimate reconstructions. Here we demonstrate how the presence of Globigerina bulloides cryptic genotypes in the Arabian Sea may influence geochemical signals of living and fossil assemblages of these morphospecies. We have identified two distinct genotypes of G. bulloides with either cool water (type-II) or warm water (type-I) temperature preferences in the Western Arabian Sea. We accompany these genetic studies with analyses of Mg/Ca and stable oxygen isotope (δ18O) compositions of individual G. bulloides shells. Both Mg/Ca and δ18O values display bimodal distribution patterns. The distribution of Mg/Ca values cannot be simply explained by seawater parameters, and we attribute it to genotype-specific biological controls on the shell geochemistry. The wide range of δ18O values in the fossil assemblage also suggests that similar controls likely influence this proxy in addition to environmental parameters. However, the magnitude of this effect on the δ18O signals is not clear from our data set, and further work is needed to clarify this. We also discuss current evidence of potential genotype-specific geochemical signals in published data on G. bulloides geochemistry and other planktonic foraminiferal species. We conclude that significant caution should be taken when utilizing G. bulloides geochemistry for paleoclimate reconstruction in the regions with upwelling activity or oceanographic fronts.

Atlantic Deep-water Response to the Early Pliocene Shoaling of the Central American Seaway

The early Pliocene shoaling of the Central American Seaway (CAS), ~4.7–4.2 million years ago (mega annum-Ma), is thought to have strengthened Atlantic Meridional Overturning Circulation (AMOC). The associated increase in northward flux of heat and moisture may have significantly influenced the evolution of Pliocene climate. While some evidence for the predicted increase in North Atlantic Deep Water (NADW) formation exists in the Caribbean and Western Atlantic, similar evidence is missing in the wider Atlantic. Here, we present stable carbon (δ13C) and oxygen (δ18O) isotope records from the Southeast Atlantic-a key region for monitoring the southern extent of NADW. Using these data, together with other δ13C and δ18O records from the Atlantic, we assess the impact of the early Pliocene CAS shoaling phase on deep-water circulation. We find that NADW formation was vigorous prior to 4.7 Ma and showed limited subsequent change. Hence, the overall structure of the deep Atlantic was largely unaffected by the early Pliocene CAS shoaling, corroborating other evidence that indicates larger changes in NADW resulted from earlier and deeper shoaling phases. This finding implies that the early Pliocene shoaling of the CAS had no profound impact on the evolution of climate.