Revisiting diagenesis on the Ontong-Java plateau: Evidence for authigenic crust precipitation in Globorotalia tumida

The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analysed to generate trace element and isotope palaeoproxy records. These sediments constitute a distinct physio-chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their palaeoceanographic interpretation. A previous study of G. tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher-Mg chamber calcite, and the preservation of a low-Mg crust on the tests significantly reduced whole-test Mg/Ca and Sr/Ca [Brown and Elderfield, 1996]. Here, we revisit these specimens with a combination of synchrotron X-ray computed tomography (sXCT) and electron probe micro-analyses (EPMA) to re-evaluate the nature of their diagenetic alteration. The dissolution of higher-Mg calcite with depth was directly observed in the sXCT data, confirming the inference of the previous study. The sXCT data further reveal a thickening of the chemically and structurally distinct calcite crust with depth. We propose that these crusts have a diagenetic origin, driven by the simultaneous dissolution of high-Mg chamber calcite and precipitation of low-Mg crust from the resulting modified pore-water solution. While the breadth of the study is limited by the nature of the techniques, the observation of both dissolution and re-precipitation of foraminiferal calcite serves to demonstrate the action of two simultaneous diagenetic alteration processes, with significant impacts on the resulting palaeoproxy signals.The authors would like to acknowledge Aleksey Sadekov, Gerald Langer, India Weidle, Alberto de Fanis, Andrew Bodey, Joan Vila-Comamala and Ulrich Wagner for their help with the project. The work was funded by the Diamond Light Source and by the ERC (2010-NEWLOG ADG-267931 grant to HE).This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/2014PA00275

Closure threat to key museum research facility

As leading representatives of the environmental and Earth science communities, we are gravely concerned about the proposed closure of the micropalaeontology research group at London's Natural History Museum ( see http://go.nature.com/KCppCe)

Holocene oscillations in temperature and salinity of the surface subpolar North Atlantic

The Atlantic meridional overturning circulation (AMOC) transports warm salty surface waters to high latitudes, where they cool, sink and return southwards at depth. Through its attendant meridional heat transport, the AMOC helps maintain a warm northwestern European climate, and acts as a control on the global climate. Past climate fluctuations during the Holocene epoch (~11,700 years ago to the present) have been linked with changes in North Atlantic Ocean circulation. The behaviour of the surface flowing salty water that helped drive overturning during past climatic changes is, however, not well known. Here we investigate the temperature and salinity changes of a substantial surface inflow to a region of deep-water formation throughout the Holocene. We find that the inflow has undergone millennial-scale variations in temperature and salinity (~3.5 °C and ~1.5 practical salinity units, respectively) most probably controlled by subpolar gyre dynamics. The temperature and salinity variations correlate with previously reported periods of rapid climate change. The inflow becomes more saline during enhanced freshwater flux to the subpolar North Atlantic. Model studies predict a weakening of AMOC in response to enhanced Arctic freshwater fluxes, although the inflow can compensate on decadal timescales by becoming more saline. Our data suggest that such a negative feedback mechanism may have operated during past intervals of climate change

Intermediate and deep water paleoceanography of the northern North Atlantic over the past 21,000 years

Benthic foraminiferal stable isotope records from four high-resolution sediment cores, forming a depth transect between 1237 m and 2303 m on the South Iceland Rise, have been used to reconstruct intermediate and deep water paleoceanographic changes in the northern North Atlantic during the last 21 ka (spanning Termination I and the Holocene). Typically, a sampling resolution of ~100 years is attained. Deglacial core chronologies are accurately tied to North Greenland Ice Core Project (NGRIP) ice core records through the correlation of tephra layers and changes in the percent abundance of Neogloboquadrina pachyderma (sinistral) with transitions in NGRIP. The evolution from the glacial mode of circulation to the present regime is punctuated by two periods with low benthic δ13C and δ18O values, which do not lie on glacial or Holocene water mass mixing lines. These periods correlate with the late Younger Dryas/Early Holocene (11.5–12.2 ka) and Heinrich Stadial 1 (14.7–16.8 ka) during which time freshwater input and sea-ice formation led to brine rejection both locally and as an overflow exported from the Nordic seas into the northern North Atlantic, as earlier reported by Meland et al. (2008). The export of brine with low δ13C values from the Nordic seas complicates traditional interpretations of low δ13C values during the deglaciation as incursions of southern sourced water, although the spatial extent of this brine is uncertain. The records also reveal that the onset of the Younger Dryas was accompanied by an abrupt and transient (~200–300 year duration) decrease in the ventilation of the northern North Atlantic. During the Holocene, Iceland-Scotland Overflow Water only reached its modern flow strength and/or depth over the South Iceland Rise by 7–8 ka, in parallel with surface ocean reorganizations and a cessation in deglacial meltwater input to the North Atlantic