Nils Andersen

155,000 Years of West African Monsoon and Ocean Thermal Evolution

A detailed reconstruction of West African monsoon hydrology over the past 155,000 years suggests a close linkage to northern high-latitude climate oscillations. Ba/Ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the Gulf of Guinea, in the eastern equatorial Atlantic (EEA), reveal centennial-scale variations of riverine freshwater input that are synchronous with northern high-latitude stadials and interstadials of the penultimate interglacial and the last deglaciation. EEA Mg/Ca-based sea surface temperatures (SSTs) were decoupled from northern high-latitude millennial-scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low-latitude solar insolation. The onset of enhanced monsoon precipitation lags behind the changes in EEA SSTs by up to 7000 years during glacial-interglacial transitions. This study demonstrates that the stadial-interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the West African monsoon dynamics through an atmospheric linkage.

Centennial scale climate instabilities in a wet early Holocene West African monsoon

A Holocene Gulf of Guinea record of riverine runoff, based on Ba/Ca in tests of a shallow-dwelling planktic foraminifer, and sea surface temperature (SST), based on Mg/Ca, reveals centennial-scale instabilities in West African monsoon (WAM) precipitation and eastern equatorial Atlantic (EEA) thermal conditions. The long-term Holocene climate trend is characterized by a warm and wet early-mid Holocene and gradual drying and cooling during the late Holocene. Superimposed on this trend are numerous centennial scale drops in precipitation during the early-mid Holocene. The greatest declines in early Holocene monsoon precipitation were accompanied by significant SST cooling in the EEA and correlate with drops in air temperature over Greenland and fresh water outbursts into the North Atlantic (NA). This observation suggests that early Holocene climate instabilities in the NA were closely linked to changes in the WAM. The strong imprint of NA events in summer monsoon precipitation suggests that these events were not confined to winter-time. The late Holocene does not show large amplitude changes in riverine runoff at the centennial level. The relatively stable late Holocene conditions likely reflect a weakening and stabilization of the monsoon system, probably due to diminished influence of the NA region due to a reduction in ice sheet.

Climatic cycles as expressed in sediments of the PROMESS1 borehole PRAD1‐2, central Adriatic, for the last 370 ka: 1. Integrated stratigraphy

[1] A multiproxy integrated chronological framework, based on oxygen and carbon stable isotope stratigraphy, biostratigraphy (foraminifera and nannoplankton bioevents and foraminifer assemblage-based climate cyclicity), magnetostratigraphy, sapropel stratigraphy, and (14)C AMS radiometric dates, has been achieved for borehole PRAD1-2, collected in 185.5 m water depth in the central Adriatic. This work was carried out within the European Community project Profiles across Mediterranean Sedimentary Systems (PROMESS1). The 71.2 m long borehole spans a time interval between late MIS 11 and MIS 1 (the last 370 ka), showing a chronological resolution of 500 and 250 years per cm during interglacial and glacial intervals, respectively. At present, this record is the most expanded and continuous marine record available for the Adriatic Basin. Several orbital cycles can be recognized in the PRAD1-2 record: the 100 ka glacial-interglacial fluctuations and the 23 ka precession-related cycles, which in turn control the deposition of sapropel layers. An integrated analysis of short-term oscillations within the Last Glaciation interval (MIS 4-MIS 2) allowed the identification of the Adriatic signature of Dansgaard-Oeschger events, showing the potential to achieve a more refined chronostratigraphic framework for the top part of the PRAD1-2 record. Finally, the age model obtained by this study allowed the chronological integration of the main foraminifera bioevents detected in the borehole as well as of the volcanoclastic layers present in the upper part of the record. Despite its proximal location, PRAD1-2 presents a continuous record and shows the potential to be consistently correlated both with deep-sea and continental records in the Mediterranean region and beyond.

Climatic cycles as expressed in sediments of the PROMESS1 borehole PRAD1-2, central Adriatic, for the last 370 ka: 2. Paleoenvironmental evolution

The multidisciplinary study of planktic and benthic foraminifera, alkenone SST, and O and C stable isotope records allowed reconstruction of the paleoenvironmental history of the central Adriatic basin over the last 360 ka B. P. In general, the main paleoclimatic changes documented in the central Adriatic appear in phase with climate change in the North Atlantic realm, except for intervals which correspond to the deposition of sapropel levels in the eastern Mediterranean. In particular, the interval between Marine Isotope Stage ( MIS) 7.5 and MIS 5 appears to be strongly influenced by the monsoonal regime. The comparison with other Mediterranean records also suggests that the Adriatic Basin was affected by very low sea surface temperature ( SST) ( down to 2 degrees C for MIS 2) during glacial intervals, which is uncommon for the Mediterranean Basin. In addition, the SST record indicates that this basin was unable to maintain warm interglacial/ interstadial conditions for durations similar to the western Mediterranean. This fact can be explained by the landlocked position and shallow depth of this basin, which make it particularly exposed to atmospheric forcing ( e. g., Siberian High) and to the strong influence of the nearby landmass during glacial intervals, producing a lag in the demise of glacial intervals. Moreover, the progressively higher values of the delta(18)O records of glacial intervals, alongside the SST record and the foraminifera assemblage, imply an increasing impact of the formation of cold and dense water since the penultimate glacial.

Carbon isotopic composition of the C37:2 alkenone in core top sediments of the South Atlantic Ocean: Effects of CO2and nutrient concentrations

[1]xa0We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone-based carbon isotopic fractionation (ep) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found ep positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43−] and [NO3−]. However, the relationship between ep and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. [1997]that the isotopic fractionation in haptophytes is related to nutrient-limited growth rates. The relatively high variability of the ep−[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth-limiting resources such as light intensity or micronutrient concentrations.

Stable Carbon Isotopic Composition of the C37:2 Alkenone: A Proxy for CO2(aq) Concentration in Oceanic Surface Waters?

We tested the applicability of the carbon isotopic composition of C37:2 alkenones (δ13C37:2) as a proxy for dissolved carbon dioxide CO2(aq) in oceanic surface waters. For this purpose we determined (δ13C37:2 in suspended particulate organic matter (POM) and surface sediments from the South Atlantic. In opposite of what would be expected from a diffusive CO2 uptake model for marine algae we observed a positive correlation between 1/[CO2(aq)] and the isotopic fractionation (ep) calculated from (δ13C37:2. This clearly demonstrates that CO2(aq) is not the primary factor controlling ep at the sites studied. On the other hand we found a negative correlation between ep and the phosphate concentration in the surface waters (0–10 m) supporting the assumption of (1997) that ep is primarily related to nutrient-limited algal growth rather than to [CO2(aq)]. Reconstructing past CO2(aq) levels from (δ13C37:2 thus requires additional proxy information in order to correct for the influence of haptophyte growth on the isotopic fractionation. In the eastern Angola Basin, we previously used δ15N of bulk organic matter as proxy for nutrient-limited growth rates. As an alternative the Sr/Ca ratio of coccoliths has been recently suggested as growth-rate proxy which should be tested in future studies.

Delta 13C measured on alkenones of surface sediment samples GeoB1008-6 to GeoB3603-1 from the South Atlantic

We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone-based carbon isotopic fractionation (Ep) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found Ep positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43-] and [NO3-]. However, the relationship between Ep and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. (1997, doi:10.1029/96GB03939) that the isotopic fractionation in haptophytes is related to nutrient-limited growth rates. The relatively high variability of the Ep-[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth-limiting resources such as light intensity or micronutrient concentrations.