Particulate and phytoplankton absorption during POLARSTERN cruises ANT-XXVI/3 and ANT-XXVIII/3

The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.

Iron (Fe) concentrations and fluxes in TALDICE ice core 0.6 to 314 ky BP

Atmospheric fluxes of iron (Fe) over the past 200 kyr are reported for the coastal Antarctic Talos Dome ice core, based on acid leachable Fe concentrations. Fluxes of Fe to Talos Dome were consistently greater than those at Dome C, with the greatest difference observed during interglacial climates. We observe different Fe flux trends at Dome C and Talos Dome during the deglaciation and early Holocene, attributed to a combination of deglacial activation of dust sources local to Talos Dome and the reorganisation of atmospheric transport pathways with the retreat of the Ross Sea ice shelf. This supports similar findings based on dust particle sizes and fluxes and Rare Earth Element fluxes. We show that Ca and Fe should not be used as quantitative proxies for mineral dust, as they all demonstrate different deglacial trends at Talos Dome and Dome C. Considering that a 20 ppmv decrease in atmospheric CO2 at the coldest part of the last glacial maximum occurs contemporaneously with the period of greatest Fe and dust flux to Antarctica, we confirm that the maximum contribution of aeolian dust deposition to Southern Ocean sequestration of atmospheric CO2 is approximately 20 ppmv.

Stable isotope record of ODP Sites 108-658 and 108-659

Ocean Drilling Program Site 658, cored below a major upwelling cell offshore Cap Blanc, contains a largely undisturbed hemipelagic sediment section spanning the Brunhes Chron and the early Quaternary and late Pliocene. The companion Site 659 recovered a complete and undisturbed Neogene profile further offshore that serves as a nonupwelling pelagic reference section. Oxygen and carbon isotope ratios in benthic (C. wuellerstorfi and in part Uvigerina sp.) and planktonic foraminifers (G. inflata) provide a climatic record of high resolution for the Brunhes Chron. At Site 658 the record extends back to the early Pleistocene and late Pliocene. The standard oxygen isotope record of the last 730,000 yr is markedly refined by a well-documented high-frequency variation (e.g., by a new "aborted" ice age at stage 13.2 and by Younger-Dryas style climatic setbacks during most terminations). In the late Pliocene, the numerical oxygen isotope stage taxonomy was extended back to stage 137 about 3.3 Ma ago. In comparison with published records, stage 114 at 2.7 Ma represents the first major glaciation event, when 18O was short-term enriched up to a middle Pleistocene glacial d18O level. About 3.17 Ma ago (stage 133), the interglacial oxygen isotope values of C. wuellerstorfi started to increase by 0.5 per mil until 2.7 Ma and then remained largely constant until the Holocene. Based on the d13C difference between C. wuellerstorfi and G. inflata, the dissolved CO2 in the ambient bottom water of Site 658 was dominated by the flux of particulate carbon from the overlying upwelling cell during the last 630,000 yr. In contrast, the advection of (upper) North Atlantic Bottom Water dominated in the control of the local CO2 content during the early Pleistocene and late Pliocene.

Log-ratio of silica to aluminium counts (ln(Si/Al)) from ODP site 108-658

Growing evidence suggests that the low atmospheric CO2 concentration of the ice ages resulted from enhanced storage of CO2 in the ocean interior, largely as a result of changes in the Southern Ocean1. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO2 release from the Southern Ocean**2, 3, 4, 5, but the mechanism connecting the North Atlantic and the Southern Ocean remains unclear. Biogenic opal export in the low-latitude ocean relies on silicate from the underlying thermocline, the concentration of which is affected by the circulation of the ocean interior. Here we report a record of biogenic opal export from a coastal upwelling system off the coast of northwest Africa that shows pronounced opal maxima during each glacial termination over the past 550,000 years. These opal peaks are consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water**2 (GNAIW). The loss of GNAIW allowed mixing with underlying silicate-rich deep water to increase the silicate supply to the surface ocean. An increase in westerly-wind-driven upwelling in the Southern Ocean in response to the North Atlantic change has been proposed to drive the deglacial rise in atmospheric CO2 (refs 3, 4). However, such a circulation change would have accelerated the formation of Antarctic intermediate water and sub-Antarctic mode water, which today have as little silicate as North Atlantic Deep Water and would have thus maintained low silicate concentrations in the Atlantic thermocline. The deglacial opal maxima reported here suggest an alternative mechanism for the deglacial CO2 release**5, 6. Just as the reduction in GNAIW led to upward silicate transport, it should also have allowed the downward mixing of warm, low-density surface water to reach into the deep ocean. The resulting decrease in the density of the deep Atlantic relative to the Southern Ocean surface promoted Antarctic overturning, which released CO2 to the atmosphere.

Contiunous surface water uncorrected and quench-corrected active fluorescence and derived chlorophyll a concentration using calibration by HPLC pigments during POLARSTERN cruise ANT-XXV/1

The relationship between phytoplankton assemblages and the associated optical properties of the water body is important for the further development of algorithms for large-scale remote sensing of phytoplankton biomass and the identification of phytoplankton functional types (PFTs), which are often representative for different biogeochemical export scenarios. Optical in-situ measurements aid in the identification of phytoplankton groups with differing pigment compositions and are widely used to validate remote sensing data. In this study we present results from an interdisciplinary cruise aboard the RV Polarstern along a north-to-south transect in the eastern Atlantic Ocean in November 2008. Phytoplankton community composition was identified using a broad set of in-situ measurements. Water samples from the surface and the depth of maximum chlorophyll concentration were analyzed by high performance liquid chromatography (HPLC), flow cytometry, spectrophotometry and microscopy. Simultaneously, the above- and underwater light field was measured by a set of high spectral resolution (hyperspectral) radiometers. An unsupervised cluster algorithm applied to the measured parameters allowed us to define bio-optical provinces, which we compared to ecological provinces proposed elsewhere in the literature. As could be expected, picophytoplankton was responsible for most of the variability of PFTs in the eastern Atlantic Ocean. Our bio-optical clusters agreed well with established provinces and thus can be used to classify areas of similar biogeography. This method has the potential to become an automated approach where satellite data could be used to identify shifting boundaries of established ecological provinces or to track exceptions from the rule to improve our understanding of the biogeochemical cycles in the ocean.

Palynology of ODP Site 108-658

Ocean Drilling Program Site 658 at 21°N off northwest Africa has a high sedimentation rate and a high concentration of pollen grains and is thus very suitable for detailed pollen analysis. The time scale for the upper 100 m (the last 670 k.y.) of Site 658 is based on biostratigraphic data and isotope stratigraphy. The pollen record has been divided into 34 zones. These are classified into 7 zone types covering a range from very arid to rather humid conditions. The sequence shows a long-term climatic decline: strong glacial stages were found only after 480 k.y. and strong interglacial stages only before 280 k.y. The Site 658 record correlates well with a terrestrial sequence from northern Greece, although both records differ in their response to global climatic change. Spectral analysis shows a 100- and a 42-k.y. period in the curves of pollen brought in by the northwest trade winds and only a 42-k.y. period in the curves of pollen mostly transported by the African Easterly Jet. A 31-k.y. period is found in the curves for Ephedra and Chenopodiaceae-Amaranthaceae. In addition, Ephedra shows a 54-k.y. period.