A global compolation of late Holocene planktonic foraminiferal d18O

We review the different sources of uncertainty affecting the oxygen isotopic composition of planktonic foraminifera and present a global planktonic foraminifera oxygen isotope data set that has been assembled within the MARGO project for the Late Holocene time slice. The data set consists of over 2100 data from recent sediment with thorough age control, that have been checked for internal consistency. We further examine how the oxygen isotopic composition of fossil foraminifera is related to hydrological conditions, based on published results on living foraminifera from plankton tows and cultures. Oxygen isotopic values (delta18O) of MARGO recent fossil foraminifera are 0.2-0.8 per mil higher than those of living foraminifera. Our results show that this discrepancy is related to the stratification of the upper water mass and generally increases at low latitudes. Therefore, as stratification of surface waters and seasonality depends on climatic conditions, the relationship between temperature and delta18O established on fossil foraminifera from recent sediment must be used with caution in paleoceanographic studies. Before models predicting seasonal flux, abundance and delta18O composition of a foraminiferal population in the sediment are available, we recommend studying relative changes in isotopic composition of fossil planktonic foraminifera. These changes primarily record variations in temperature and oxygen isotopic composition of sea water, although part of the changes might reflect modifications of planktonic foraminifera seasonality or depth habitat

Solid phase phosphorus geochemistry in surface sediments of sediment cores GeoB3718-4, GeoB3702-2 and GeoB3707-4

In this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the Namibian coastal upwelling area. Examination of the sediments reveals that organic and biogenic phosphorus are the major P-containing phases preserved. High Corg/Porg ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has either already occurred on the suspension load or that the organic matter deposited at these sites is already rather refractory. Release of phosphate in the course of benthic microbial organic matter degradation cannot be identified as the dominating process within the observed internal benthic phosphorus cycle. Dissolved phosphate and iron in the pore water are closely coupled, showing high concentrations below the oxygenated surface layer of the sediments and low concentrations at the sediment-water interface. The abundant presence of Fe(III)-bound phosphorus in the sediments document the co-precipitation of both constituents as P-containing iron (oxyhydr)oxides. However, highly dissolved phosphate concentrations in pore waters cannot be explained, neither by simple mass balance calculations nor by the application of an established computer model. Under the assumption of steady state conditions, phosphate release rates are too high as to be balanced with a solid phase reservoir. This discrepancy points to an apparent lack of solid phase phosphorus at sediment depth were suboxic conditions prevail. We assume that the known, active, fast and episodic particle mixing by burrowing macrobenthic organisms could repeatedly provide the microbially catalyzed processes of iron reduction with authigenic iron (oxyhydro)oxides from the oxic surface sediments. Accordingly, a multiple internal cycling of phosphate and iron would result before both elements are buried below the iron reduction zone.

(Table 1) Two-way travel times and depths of picked internal reflection horizons (IRHs) of the NGRIP ice core

During the summer of 2003, a ground-penetrating radar survey around the North Greenland Icecore Project (NorthGRIP) deep ice-core drilling site (75°06' N, 42°20' W; 2957 m a.s.l.) was carried out using a shielded 250 MHz radar system. The drill site is located on an ice divide, roughly 300 km north-northwest of the summit of the Greenland ice sheet. More than 430 km of profiles were measured, covering a 10 km by 10 km area, with a grid centered on the drilling location, and eight profiles extending beyond this grid. Seven internal horizons within the upper 120 m of the ice sheet were continuously tracked, containing the last 400 years of accumulation history. Based on the age-depth and density-depth distribution of the deep core, the internal layers have been dated and the regional and temporal distribution of accumulation rate in the vicinity of NorthGRIP has been derived. The distribution of accumulation shows a relatively smoothly increasing trend from east to west from 145 kg/m**2/a to 200 kg/m**2/a over a distance of 50 km across the ice divide. The general trend is overlain by small-scale variations on the order of 2.5 kg/m**2/a/km, i.e. around 1.5% of the accumulation mean. The temporal variations of the seven periods defined by the seven tracked isochrones are on the order of +-4% of the mean of the last 400 years, i.e. at NorthGRIP ±7 kg/m**2/a. If the regional accumulation pattern has been stable for the last several thousand years during the Holocene, and ice flow has been comparable to today, advective effects along the particle trajectory upstream of NorthGRIP do not have a significant effect on the interpretation of climatically induced changes in accumulation rates derived from the deep ice core over the last 10 kyr.

Observations on the propagation of internal tidal waves on the continental slope off Sierra Leone

During R/V Meteor-cruise no. 30 4 moorings with 17 current meters were placed on the continental slope of Sierra Leone at depths between 81 and 1058 meters. The observation period started on March 8, 1973, 16.55 hours GMT and lasted 19 days for moorings M30_068MOOR, M30_069MOOR, M30_070MOOR on the slope and 9 days for M30_067MOOR on the shelf. One current meter recorded at location M30_067MOOR for 22 days. Hydrographic data were collected at 32 stations by means of the "Kieler Multi-Meeressonde". Harmonic analysis is applied to the first 15 days of the time series to determine the M2 and S2 tides. By vertically averaging of the Fourier coefficients the field of motion is separated into its barotropic and its baroclinic component. The expected error generated by white Gaussian noise is estimated. To estimate the influence of the particular vertical distribution of the current meters, the barotropic M2 tide is calculated by ommitting and interchanging time series of different moorings. It is shown that only the data of moorings M30_069MOOR, M30_070MOOR and M30_067MOOR can be used. The results for the barotropic M2 tide agree well with the previous publications of other authors. On the slope at a depth of 1000 m there is a free barotropic wave under the influence of the Coriolis-force propagating along the slope with an amplitude of 3.4 cm S**-1. On the shelf, the maximum current is substantially greater (5.8 cm s**-1) and the direction of propagation is perpendicular to the slope. As for the continental slope a separation into different baroclinic modes using vertical eigenmodes is not reasonable, an interpretation of the total baroclinic wave field is tried by means of the method of characteristis. Assuming the continental slope to generate several linear waves, which superpose, baroclinic tidal ellipses are calculated. The scattering of the direction of the major axes M30_069MOOR is in contrast to M30_070MOOR, where they are bundled within an angle of 60°. This is presumably caused by the different character of the bottom topography in the vicinity of the two moorings. A detailed discussion of M30_069MOOR is renounced since the accuracy of the bathymetric chart is not sufficient to prove any relation between waves and topography. The bundeling of the major axes at M30_070MOOR can be explained by the longslope changes of the slope, which cause an energy transfer from the longslope barotropic component to the downslope baroclinic component. The maximum amplitude is found at a depth of 245 m where it is expected from the characteristics originating at the shelf edge. Because of the dominating barotropic tide high coherence is found between most of the current meters. To show the influence of the baroclinic tidal waves, the effect of the mean current is considered. There are two periods nearly opposite longshore mean current. For 128 hours during each of these periods, starting on March 11, 05.00, and March 21, 08.30, the coherences and energy spectra are calculated. The changes in the slope of the characteristics are found in agreement with the changes of energy and coherence. Because of the short periods of nearly constant mean current, some of the calculated differences of energy and coherence are not statistically significant. For the M2 tide a calculation of the ratios of vertically integrated total baroclinic energy and vertically integrated barotropic kinetic energy is carried out. Taking into account both components (along and perpendicular to the slope) the obtained values are 0.75 and 0.98 at the slope and 0.38 at the shelf. If each component is considered separately, the ratios are 0.39 and 1.16 parallel to the slope and 5.1 and 15.85 for the component perpendicular to it. Taking the energy transfer from the longslope component to the doenslope component into account, a simple model yields an energy-ratio of 2.6. Considering the limited application of the theory to the real conditions, the obtained are in agreement with the values calculated by Sandstroem.

Two-way traveltimes of internal layers and ice thickness between Dome C - Vostok - Dome A

Chinese scientists will start to drill a deep ice core at Kunlun station near Dome A in the near future. Recent work has predicted that Dome A is a location where ice older than 1 million years can be found. We model flow, temperature and the age of the ice by applying a three-dimensional, thermomechanically coupled full-Stokes model to a 70 × 70 km**2 domain around Kunlun station, using isotropic non-linear rheology and different prescribed anisotropic ice fabrics that vary the evolution from isotropic to single maximum at 1/3 or 2/3 depths. The variation in fabric is about as important as the uncertainties in geothermal heat flux in determining the vertical advection which in consequence controls both the basal temperature and the age profile. We find strongly variable basal ages across the domain since the ice varies greatly in thickness, and any basal melting effectively removes very old ice in the deepest parts of the subglacial valleys. Comparison with dated radar isochrones in the upper one third of the ice sheet cannot sufficiently constrain the age of the deeper ice, with uncertainties as large as 500 000 years in the basal age. We also assess basal age and thermal state sensitivities to geothermal heat flux and surface conditions. Despite expectations of modest changes in surface height over a glacial cycle at Dome A, even small variations in the evolution of surface conditions cause large variation in basal conditions, which is consistent with basal accretion features seen in radar surveys.

Seawater carbonate chemistry and calcification during experiments with a coral Madracis auretenra, 2010

Physiological data and models of coral calcification indicate that corals utilize a combination of seawater bicarbonate and (mainly) respiratory CO2 for calcification, not seawater carbonate. However, a number of investigators are attributing observed negative effects of experimental seawater acidification by CO2 or hydrochloric acid additions to a reduction in seawater carbonate ion concentration and thus aragonite saturation state. Thus, there is a discrepancy between the physiological and geochemical views of coral biomineralization. Furthermore, not all calcifying organisms respond negatively to decreased pH or saturation state. Together, these discrepancies suggest that other physiological mechanisms, such as a direct effect of reduced pH on calcium or bicarbonate ion transport and/or variable ability to regulate internal pH, are responsible for the variability in reported experimental effects of acidification on calcification. To distinguish the effects of pH, carbonate concentration and bicarbonate concentration on coral calcification, incubations were performed with the coral Madracis auretenra (= Madracis mirabilis sensu Wells, 1973) in modified seawater chemistries. Carbonate parameters were manipulated to isolate the effects of each parameter more effectively than in previous studies, with a total of six different chemistries. Among treatment differences were highly significant. The corals responded strongly to variation in bicarbonate concentration, but not consistently to carbonate concentration, aragonite saturation state or pH. Corals calcified at normal or elevated rates under low pH (7.6-7.8) when the seawater bicarbonate concentrations were above 1800 µm. Conversely, corals incubated at normal pH had low calcification rates if the bicarbonate concentration was lowered. These results demonstrate that coral responses to ocean acidification are more diverse than currently thought, and question the reliability of using carbonate concentration or aragonite saturation state as the sole predictor of the effects of ocean acidification on coral calcification.