Solar radiation over and under sea ice and photographic quantification of Ice algal aggregates during the POLARSTERN cruise ARK-XXVII/3 (IceArc) in summer 2012

The ice cover of the Arctic Ocean has been changing dramatically in the last decades and the consequences for the sea-ice associated ecosystem remain difficult to assess. Algal aggregates underneath sea ice have been described sporadically but the frequency and distribution of their occurrence is not well quantified. We used upward looking images obtained by a remotely operated vehicle (ROV) to derive estimates of ice algal aggregate biomass and to investigate their spatial distribution. During the IceArc expedition (ARK-XXVII/3) of RV Polarstern in late summer 2012, different types of algal aggregates were observed floating underneath various ice types in the Central Arctic basins. Our results show that the floe scale distribution of algal aggregates in late summer is very patchy and determined by the topography of the ice underside, with aggregates collecting in dome shaped structures and at the edges of pressure ridges. The buoyancy of the aggregates was also evident from analysis of the aggregate size distribution. Different approaches used to estimate aggregate biomass yield a wide range of results. This highlights that special care must be taken when upscaling observations and comparing results from surveys conducted using different methods or on different spatial scales.

(Table 1) Composition of DSDP Hole 37-334 clinopyroxenes

Recent studies of abyssal peridotites (Johnson et al., 1990, doi:10.1029/JB095iB03p02661), mid-ocean-ridge basalts (MORBs) (McKenzie, 1985, doi:10.1016/0012-821X(85)90001-9) and their entrained melt inclusions (Sobolev and Shimizu, 1993, doi:10.1038/363151a0; Humler and Whitechurch, 1988, doi:10.1016/0012-821X(88)90055-6) have shown that fractional melting of the upwelling sub-oceanic mantle produces magmas with a much wider range of compositions than erupted MORBs. In particular, it seems that strongly depleted primary magmas are routinely produced by melting beneath ridges (Johnson et al., 1990, doi:10.1029/JB095iB03p02661). The absence of strongly depleted melts as erupted lavas prompts the question of how long such magmas survive beneath ridges, before their distinctive compositions are concealed by mixing with more enriched magmas. Here we report mineral compositions from a unique suite of oceanic cumulates recovered from DSDP Site 334 (Aumento et al., doi:10.2973/dsdp.proc.37.1977), which indicate that the rocks crystallized from basaltic liquids that were strongly depleted in Na, Ti, Zr, Y, Sr and rare-earth elements relative to any erupted MORB. It thus appears that the magmatic plumbing system beneath the Mid-Atlantic Ridge permitted strongly depleted magmas to accumulate in a magma chamber and remain sufficiently isolated to produce cumulate rocks. Even so, spatial heterogeneity in the compositions of high-calcium pyroxenes suggests that in the later stages of solidification these rocks reacted with infiltrating enriched basaltic liquids.

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.

Major and trace element abundances of igneous rocks from DSDP Site 37-334

REE abundances in gabbros and peridotites from Site 334 of DSDP Leg 37 show that these rocks are cumulates produced by fractional crystallization of a primitive oceanic tholeiite magma. They may be part of a layered oceanic complex. The REE distributions in the residual liquids left after such a fractionation are similar to those of incompatible element-depleted oceanic tholeiites. The REE data indicate that the basalts which overlie the gabbro-peridotite complex, are not genetically related to plutonic rocks.

Chemical composition of bottom sediments and rocks from DSDP Holes 3-14 through 64-477

Data on analyses of chemical composition of DSDP samples of bottom sediments and rocks carried out in P.P. Shirshov Institute of Oceanology are reported. Basal sediments and sedimentary rocks prevail in the sample set.