Isothermal remanent magnetization of Greenland ice: Preliminary results

The magnetic mineral content of wind“transported dust should reflect atmospheric transport dynamics and conditions in its source areas, and could thus be used as an environmental proxy. To test the feasibility of determining the magnetic mineral content in polar ice, isothermal remanent magnetization (IRM) was measured on a small suite of Greenland ice samples of Holocene (interglacial) and Last Glacial Maximum (LGM) age. Although the extremely low dust concentrations limit weak field (susceptibility) measurements, all samples contained an easily measurable concentration of magnetic minerals that can be estimated using IRM intensity provided that special precautions are used. IRM experiments at liquid nitrogen temperatures indicate ice magnetic properties which are consistent with that expected from varying concentrations of magnetite or maghemite. Interestingly, the Holocene ice samples tend to have higher magnetic concentrations, despite having much lower total polar dust contents, than the few LGM ice samples tested thus far

Reconstruction of east–west deep water exchange in the low latitude Atlantic Ocean over the past 25,000 years

Radiogenic neodymium isotopes have been used as a water mass mixing proxy to investigate past changes in ocean circulation. Here we present a new depth transect of deglacial neodymium isotope records measured on uncleaned planktic foraminifera from five cores spanning from 3300 to 4900 m on the Mauritanian margin, in the tropical eastern Atlantic as well as an additional record from 4000 m on the Ceara Rise in the equatorial western Atlantic. Despite being located under the Saharan dust plume, the eastern Atlantic records differ from the composition of detrital inputs through time and exhibit similar values to the western Atlantic foraminiferal Nd across the deglaciation. Therefore we interpret the foraminiferal values as recording deep water Nd isotope changes. All six cores shift to less radiogenic values across the deglaciation, indicating that they were bathed by a lower proportion of North Atlantic Deep Water during the Last Glacial Maximum (LGM) relative to the Holocene. The eastern Atlantic records also show that a neodymium isotope gradient was present during the LGM and during the deglaciation, with more radiogenic values observed at the deepest sites. A homogeneous water mass observed below 3750 m in the deepest eastern Atlantic during the LGM is attributed to the mixing of deep water by rough topography as it passes from the western Atlantic through the fracture zones in the Mid-Atlantic Ridge. This implies that during the LGM the low latitude deep eastern Atlantic was ventilated from the western Atlantic via advection through fracture zones in the same manner as occurs in the modern ocean. Comparison with carbon isotopes indicates there was more respired carbon in the deep eastern than deep western Atlantic during the LGM, as is also seen in the modern Atlantic Ocean

Influence of Patagonian glaciers on Antarctic dust deposition during the last glacial period

Dust in the atmosphere plays a role in the transparency of the atmosphere1, the mineral nourishment of the oceans and can be used to constrain global circulation models today and in the past. Antarctic ice cores provide an 800,000 year record of changes in dust flux thought to reflect changes in the vigour of global atmospheric circulation and environmental conditions in source areas. Here for the first time we link the source of Last Glacial dust peaks in Antarctica to the gravel outwash plains of Patagonian glaciers in the Magellan area of southernmost South America. We find that there is an on-off switch in that the peaks coincide with episodes when glaciers discharge sediment directly onto outwash plains but not when they terminate in lakes. This finding helps solve several long-standing puzzles, namely: why both dust and fresh water diatom concentrations during glacial maxima are so much higher (x ~20) than at the present day; why dust peaks occur only below a certain temperature threshold; and why the decline in dust concentrations at the end of glacial cycles precedes the main phase of warming, the rise in sea level, and the reduction in southern hemisphere sea ice extent

The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data

Neodymium isotopic compositions (143Nd/144Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years). At water depths ≥ 1500 m, property-property plots show clear correlations between seawater εNd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater εNd distribution. At ≥ 200 m, basin-scale seawater T-S-εNd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive εNd values are compared using property-property plots and T-S-εNd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥ 1500 m) seawater data, we established empirical equations that predict the main, large-scale, deepwater εNd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive εNd is observed mainly within 1000 km from the continents. Generally, seawater and archive εNd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥ 600 m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths

Transport of airborne lithogenic material down through the water column in two contrasting regions of the eastern subtropical North Atlantic Ocean

International audienceDownward particle fluxes were measured using deep-moored sediment traps deployed in two regions of contrasting primary productivity levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. The high percentage of lithogenic material (~20-30% on average) in the particulate matter collected shows the regional significance of the atmospheric dust inputs originating from West Africa. The magnitudes of lithogenic and biogenic fluxes decrease ~5-6 and-•8-9 fold, respectively, from near the African margin (mesotrophic region) to the remote open ocean (oligotrophic region). These trophic differences seem to give rise to differences in the characteristics of the downward transport of lithogenic material. At the oligotrophic site, the relatively low and slow export of biogenic matter apparently limits and delays the removal of lithogenic particles delivered to surface waters from the atmosphere. In contrast, the higher biological activity in the mesotrophic region seems to provide persistent conditions for an efficient and faster downward transport of the deposited lithogenic particles, and the temporal variability of lithogenic fluxes largely reflects that of the atmospheric dust inputs. Thus whether the temporal variability of the exported lithogenic flux in the water column follows that of the atmospheric deposition appears to depend on the trophic status. In the mesotrophic region the oft-observed linear relationship between lithogenic and particulate organic matter (hereinafter POM) fluxes breaks down at high POM fluxes. This observation adds weight to the idea that linear relationships between POM fluxes and some candidate proxies for POM transfer cannot be assumed when POM export is large. A high mesoscale variability of biogenic, but not lithogenic, fluxes in the water column of the mesotrophic region underscores the relevance of mesoscale studies for regional estimates of export of biogenic material

Elemental and isotopic (Sr and Nd) characterization of deposited dust on the Senegalese margin and implications for Saharan dust source geochemical fingerprinting

info:eu-repo/semantics/publishe