Partial melting experiments on oceanic cumulated gabbros

We performed hydrous partial melting experiments at shallow pressures (0.2 GPa) under slightly oxidizing conditions (NNO oxygen buffer) on oceanic cumulate gabbros drilled by ODP (Ocean Drilling Program) cruises to evaluate whether the partial melting of oceanic gabbro can generate SiO2-rich melts with compositions typical of oceanic plagiogranites. The experimental melts of the low-temperature runs broadly overlap those of natural plagiogranites. At 940 °C, the normalized SiO2 contents of the experimental melts of all systems range between 60 and 61 wt%, and at 900 °C between 63 and 68 wt%. These liquids are characterized by low TiO2 and FeOtot contents, similar to those of natural plagiogranites from the plutonic section of the oceanic crust, but in contrast to Fe and Ti-rich low-temperature experimental melts obtained in MORB systems at ~950 °C. The ~1,500-m-long drilled gabbroic section of ODP Hole 735B (Legs 118 and 176) at the Southwest Indian Ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high-temperature shearing. The compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins, while the compositions of the crystals of low-temperature runs correspond to those of minerals from high-temperature microscopic veins occurring in the gabbroic section of the Hole 735B. This suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water-rich fluid phase. If the temperature estimations for hightemperature shear zones are correct (up to 1,000 °C), and a water-rich fluid phase is present, the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust.

Beryllium ages, sedimentology and weathering characteristics of Reedy Glacier deposits

Deposits corresponding to multiple periods of glaciation are preserved in ice-free areas adjacent to Reedy Glacier, southern Transantarctic Mountains. Glacial geologic mapping, supported by 10Be surface-exposure dating, shows that Reedy Glacier was significantly thicker than today multiple times during the mid-to-late Cenozoic. Longitudinal-surface profiles reconstructed from the upper limits of deposits indicate greater thickening at the glacier mouth than at the head during these episodes, indicating that Reedy Glacier responded primarily to changes in the thickness of the West Antarctic Ice Sheet. Surface-exposure ages suggest this relationship has been in place since at least 5 Ma. The last period of thickening of Reedy Glacier occurred during Marine Isotope Stage 2, at which time the glacier surface near its confluence with the West Antarctic Ice Sheet was at least 500 m higher than today.

(Table 1) Vertical turbulent eddy diffusivity of selected stations from the Nathaniel B. Palmer cruise NBP09-01

Dissolved iron (DFe) and total dissolvable Fe (TDFe) were measured in January-February 2009 in Pine Island Bay, as well as in the Pine Island and Amundsen polynyas (Amundsen Sea, Southern Ocean). Iron (Fe) has been shown to be a limiting nutrient for phytoplankton growth, even in the productive continental shelves surrounding the Antarctic continent. However, the polynyas of the Amundsen Sea harbor the highest concentrations of phytoplankton anywhere in Antarctica. Here we present data showing the likely sources of Fe that enable such a productive and long lasting phytoplankton bloom. Circumpolar Deep Water (CDW) flows over the bottom of the shelf into the Pine Island Bay where DFe and TDFe were observed to increase from 0.2 to 0.4 nM DFe and from 0.3-4.0 to 7-14 nM TDFe, respectively. At the southern end of Pine Island Bay, the CDW upwelled under the Pine Island Glacier, bringing nutrients (including Fe) to the surface and melting the base of the glacier. Concentrations of DFe in waters near the Pine Island Glacier and the more westward lying Crosson, Dotson, and Getz Ice Shelves varied between 0.40 and 1.31 nM, depending on the relative magnitude of upwelling, turbulent mixing, and melting. These values represent maximum concentrations since associated ligands (which increase the solubility of Fe in seawater) were saturated with Fe (Thuroczy et al., 2012, doi:10.1016/j.dsr2.2012.03.009). The TDFe concentrations were very high compared to what previously has been measured in the Southern Ocean, varying between 3 and 106 nM. In the Pine Island Polynya, macronutrients and DFe were consumed by the phytoplankton bloom and concentrations were very low. We calculate that atmospheric dust contributed < 1% of the Fe necessary to sustain the phytoplankton bloom, while vertical turbulent eddy diffusion from the sediment, sea ice melt, and upwelling contributed 1.0-3.8%, 0.7-2.9%, and 0.4-1.7%, respectively. The largest source was Fe input from the PIG, which could satisfy the total Fe demand by the phytoplankton bloom by lateral advection of Fe over a range of 150 km from the glacier. The role of TDFe as a phytoplankton nutrient remains unclear, perhaps representing an important indirect Fe source via dissolution and complexation by dissolved organic ligands (Gerringa et al., 2000, doi:10.1016/S0304-4203(99)00092-4; Borer et al., 2005, doi:10.1016/j.marchem.2004.08.006).