Mineralogy of sea-floor surface sediments, Sonne cruise SO202/1 to the North Pacific and Bering Sea

During expedition 202 of research vessel SONNE in 2009, 39 sea-floor surface sediments were sampled over a wide area across the North Pacific and the Bering Sea, which are well suited as reference archives of modern environmental processes. In this study, we used the samples to infer the documentation of land-ocean linkages of terrigenous sediment supply. We followed an integrated approach of grain-size analysis, bulk mineralogy, and clay mineralogy in combination with statistical data evaluation (end-member modelling of grain-size data, fuzzy-cluster analysis of mineralogical data), in order to identify the significant sources and modes of sediment transport in an overregional context. We also compiled literature data on clay mineralogy and updated those with the new data. Today, two processes of terrigenous sediment supply prevail in the study area: far-distant aeolian sediment supply to the pelagic North Pacific as well as hemipelagic sediment dispersal from nearby land sources by ocean currents along the continental margins and island arcs of the study area. The aeolian particles show the finest grain sizes (clay and fine silt), while the hemipelagic sediments have high abundances of sortable silt, particles >10 microns.

Chemistry and grain-size distribution of terrigenous sediments deposited at Site GeoB9501

The Sahara Desert is the largest source of mineral dust in the world. Emissions of African dust increased sharply in the early 1970s, a change that has been attributed mainly to drought in the Sahara/Sahel region caused by changes in the global distribution of sea surface temperature. The human contribution to land degradation and dust mobilization in this region remains poorly understood, owing to the paucity of data that would allow the identification of long-term trends in desertification. Direct measurements of airborne African dust concentrations only became available in the mid-1960s from a station on Barbados and subsequently from satellite imagery since the late 1970s: they do not cover the onset of commercial agriculture in the Sahel region ~170 years ago. Here we construct a 3,200-year record of dust deposition off northwest Africa by investigating the chemistry and grain-size distribution of terrigenous sediments deposited at a marine site located directly under the West African dust plume. With the help of our dust record and a proxy record for West African precipitation we find that, on the century scale, dust deposition is related to precipitation in tropical West Africa until the seventeenth century. At the beginning of the nineteenth century, a sharp increase in dust deposition parallels the advent of commercial agriculture in the Sahel region. Our findings suggest that human-induced dust emissions from the Sahel region have contributed to the atmospheric dust load for about 200 years.

He isotope, U/Th isotope, Calcium carbonate, biogenic opal, rare earth element concentrations, and grain size distribution measured on core-top sediments during SONNE cruise SO202 (INOPEX)

Eolian dust is a significant source of iron and other nutrients that are essential for the health of marine ecosystems and potentially a controlling factor of the high nutrient-low chlorophyll status of the Subarctic North Pacific. We map the spatial distribution of dust input using three different geochemical tracers of eolian dust, 4He, 232Th and rare earth elements, in combination with grain size distribution data, from a set of core-top sediments covering the entire Subarctic North Pacific. Using the suite of geochemical proxies to fingerprint different lithogenic components, we deconvolve eolian dust input from other lithogenic inputs such as volcanic ash, ice-rafted debris, riverine and hemipelagic input. While the open ocean sites far away from the volcanic arcs are dominantly composed of pure eolian dust, lithogenic components other than eolian dust play a more crucial role along the arcs. In sites dominated by dust, eolian dust input appears to be characterized by a nearly uniform grain size mode at ~4 µm. Applying the 230Th-normalization technique, our proxies yield a consistent pattern of uniform dust fluxes of 1-2 g/m**2/yr across the Subarctic North Pacific. Elevated eolian dust fluxes of 2-4 g/m**2/yr characterize the westernmost region off Japan and the southern Kurile Islands south of 45° N and west of 165° E along the main pathway of the westerly winds. The core-top based dust flux reconstruction is consistent with recent estimates based on dissolved thorium isotope concentrations in seawater from the Subarctic North Pacific. The dust flux pattern compares well with state-of-the-art dust model predictions in the western and central Subarctic North Pacific, but we find that dust fluxes are higher than modeled fluxes by 0.5-1 g/m**2/yr in the northwest, northeast and eastern Subarctic North Pacific. Our results provide an important benchmark for biogeochemical models and a robust approach for downcore studies testing dust-induced iron fertilization of past changes in biological productivity in the Subarctic North Pacific.