Decadal-centennial scale monsoon variations in the Arabian Sea during the Early Holocene

An essential prerequisite for the prediction of future climate change due to anthropogenic input is an understanding of the natural processes that control Earth's climate on timescales comparable to human-lifespan. The Early Holocene period was chosen to study the natural climate variability in a warm interval when solar insolation was at its maximum. The monsoonal system of the Tropics is highly sensitive to seasonal variations in solar insolation, and consequently marine sediments from the region are a potential monitor of past climate change. Here we show that during the Early Holocene period rapid

Multidecadal variations in the early Holocene outflow of Red Sea Water into the Arabian Sea

We present Holocene stable oxygen isotope data from the deep Arabian Sea off Somalia at a decadal time resolution as a proxy for the history of intermediate/upper deep water. These data show an overall δ18O reduction by 0.5‰ between 10 and ~6.5 kyr B.P. superimposed upon short-term δ18O variations at a decadal-centennial timescale. The amplitude of the decadal variations is 0.3‰ prior, and up to 0.6‰ subsequent, to ~8.1 kyr B.P. We conclude from modeling experiments that the short-term δ18O variations between 10 and ~6.5 kyr B.P. most likely document changes in the evaporation-precipitation balance in the central Red Sea. Changes in water temperature and salinity cause the outflowing Red Sea Water to settle roughly 800 m deeper than today

Planktic foraminiferal shell thinning in the Arabian Sea due to anthropogenic ocean acidification?

About one third of the anthropogenic carbon dioxide (CO<sub>2</sub>) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO<sub>2</sub> invades the surface ocean, carbonate ion concentrations and pH are lowered. Laboratory studies indicate that this reduces the calcification rates of marine calcifying organisms, including planktic foraminifera. Such a reduction in calcification resulting from anthropogenic CO<sub>2</sub> emissions has not been observed, or quantified in the field yet. Here we present the findings of a study in the Western Arabian Sea that uses shells of the surface water dwelling planktic foraminifer <i>Globigerinoides ruber</i> in order to test the hypothesis that anthropogenically induced acidification has reduced shell calcification of this species. We found that light, thin-walled shells from the surface sediment are younger (based on <sup>14</sup>C and δ<sup>13</sup>C measurements) than the heavier, thicker-walled shells. Shells in the upper, bioturbated, sediment layer were significantly lighter compared to shells found below this layer. These observations are consistent with a scenario where anthropogenically induced ocean acidification reduced the rate at which foraminifera calcify, resulting in lighter shells. On the other hand, we show that seasonal upwelling in the area also influences their calcification and the stable isotope (δ<sup>13</sup>C and δ<sup>18</sup>O) signatures recorded by the foraminifera shells. Plankton tow and sediment trap data show that lighter shells were produced during upwelling and heavier ones during non-upwelling periods. Seasonality alone, however, cannot explain the <sup>14</sup>C results, or the increase in shell weight below the bioturbated sediment layer. We therefore must conclude that probably both the processes of acidification and seasonal upwelling are responsible for the presence of light shells in the top of the sediment and the age difference between thick and thin specimens

Surface water temperature, salinity, and density changes in the northeast Atlantic during the last 45,000 years: Heinrich events, deep water formation, and climatic rebounds

We developed a new method to calculate sea surface salinities (SSS) and densities (SSD) from planktonic foraminiferal delta(18)O and sea surface temperatures (SST) as determined from planktonic foraminiferal species abundances. SST, SSS, and SSD records were calculated for the last 45,000 years for Biogeochemical Oceanic Flux Study (BOFS) cores 5K and 8K recovered from the northeast Atlantic. The strongest feature is the dramatic drop in all three parameters during the Heinrich ''ice-rafting'' events. We modelled the possibility of deepwater formation in the northeast Atlantic from the SSD records, by assuming that the surface waters at our sites cooled as they flowed further north. Comparison with modelled North Atlantic deepwater densities indicates that there could have been periods of deepwater formation between 45,000 and 30,000 C-14 years B.P. (interrupted by iceberg meltwater input of Heinrich event 3 and 4, at 27,000 and 38,000 C-14 years B.P.) and during the Holocene. No amount of cooling in the northeast Atlantic between 30,000 and 13,000 years could cause deep water to form, because of the low salinities resulting from the high meltwater inputs from icebergs. Our records indicate that after each Heinrich event there were periods of climatic rebound, with milder conditions persisting for up to 2000 years, as indicated by the presence of warmer and more saline water masses. After these warm periods conditions returned to average glacial levels. These short term cold and warm episodes in the northeast Atlantic ate superimposed on the general trend towards colder conditions of the Last Glacial Maximum (LGM). Heinrich event 1 appears to be unique as it occurs as insolation rose and was coeval with the initial melting of the Fennoscandian ice sheet. We propose that meltwater input of Heinrich event 1 significantly reduced North Atlantic Deep Water formation reducing the heat exchange between the low and high latitudes, thus delaying deglaciation by about 1500 radiocarbon years (2000 calendar years)

Evidence for early warming and cooling in North Atlantic surface waters during the last interglacial

In-depth analysis of planktic foraminiferal census data paired with δ18O records of specific indicator species provides new insight into the surface ocean evolution of the northeast Atlantic during the previous interglacial warm period (oxygen isotope stage (OIS) 5e). Full interglacial conditions existed at the study site for a maximum of only 8 kyr, between 125 and 117 ka. Highest sea surface temperatures (SSTs) occurred during early OIS 5e concomitant with high summer insolation but after the main phase of ice sheet melting of the preceding glaciation (Saalian). This early peak SST interval is marked by the appearance of tropical-subtropical species and lasted for 4 kyr until 121 ka, as corroborated by a major change in planktic δ18O. Relative stability in global ice volume continued for another 3–4 kyr before SSTs dropped further toward the next stadial. During early OIS 5e the situation of the surface water vertical structure appears to have been different from the early Holocene. For OIS 5e it is therefore suggested that the particular melting history of late Saalian ice had a long-lasting and profound effect on both postdeglacial surface water mass configuration in the North Atlantic and heat-moisture transfer into Europe

The structure of Termination II (penultimate deglaciation and Eemian) in the North Atlantic.

A study of the 140-100 ka interval in core T90-9P from the North Atlantic (45°N, 25°W), based on analysis of oxygen and carbon isotope records from planktonic and benthonic foraminifera, and from the bulk sediment fine fraction facilitates a detailed paleoceanographic reconstruction of the penultimate deglaciation (Termination II), and of the Eemian interglacial (

Effect of supra-lysoclinal dissolution on oxygen isotopes and Mg/Ca in planktonic foraminifera

Mg/Ca in planktonic foraminiferal tests is now well established as a temperature proxy for surface and near-surface waters. The advantage of using Mg/Ca coupled with oxygen isotope ratio measured on the same foraminiferal sample provides the possibility to determine seawater d18O. There has been concern, however, about the post-mortem processes (e.g., dissolution) affecting foraminiferal Mg/Ca and d18O. We document the effect of dissolution on both Mg/Ca and d18O of several planktonic foraminiferal species along two depth transects (a) ~450-4000 m off Somalia and (b) ~1500-4500 m from Ontong Java Plateau. The planktonic foraminiferal species, Globigerinoides ruber, Globigerinoides sacculifer, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei and Globorotalia tumida were analysed for Mg/Ca and d18O from core-top samples along the depth transects within a narrow size range (e.g., 250-300, 355-425 um). Mg/Ca and d18O in shells of surface dwelling planktonic foraminiferal species (e.g., G. ruber and G. sacculifer) show a small decrease while Mg/Ca and d18O of the deeper dwelling species (e.g., N. dutertrei and G. tumid) show more pronounced effect of supralysoclinal dissolution. The effect of supralysoclinal dissolution has an insignificant effect on the reconstruction of seawater d18O based on surface dwelling planktonic species compared to the deeper dwelling species

Water mass evolution at intermediate depths off somalia for the past 31 kyr based on the benthic foraminiferal chemistry

Studying past changes in temperature and salinity of bottom water is of interest because they reflect not only regional expressions, but also global changes in heat flux and circulation patterns at depth. Here we present the results of sediments off Somalia to identify the water mass distribution present at the intermediate depths in the Arabian Sea for the past 31 kyr using paired Mg/Ca and d18O measurements of epi-benthic foraminifer Cibicidoides kullenbergi. First, we calibrate Mg/Ca in C. kullenbergi and Cibicidoides wuellerstorfi with measured in situ bottom water and isotopic temperatures from core-tops along a depth transect off Somalia. We compare the new data with the published calibrations and discuss other potential effects (e.g., carbonate ion). Second, we investigate temperature and salinity changes from the coupled d18O and Mg/Ca records of C. kullenbergi for the past 31 kyr off Somalia at ~1580 m water depth. The results show that the glacial water mass at the intermediate depth was warmer and more saline compared to the present day. We suggest that this water mass is Glacial Arabian Sea Intermediate Water (GASIW) and bathed the slope at intermediate levels offshore Somalia during the last glacial. The GASIW is most likely to be originated in the northern Arabian Sea although a possible influence of glacial Red Sea water can not be excluded