Eolian dust input to the Subarctic North Pacific
Introduction
Eolian dust is a major driver in the global climate system through its influence on light scattering and absorption (Harrison et al., 2001, Tegen and Fung, 1994), cloud and precipitation properties (Kaufman et al., 2002, Levin et al., 1996), and the oceanic biogeochemical cycles of carbon and nutrients as a result of delivering micronutrients like iron (Duce and Tindale, 1991, Jickells et al., 2005, Martin, 1990).
The Subarctic North Pacific (SNP) is one of the three principal High Nutrient-Low Chlorophyll (HNLC) regions of the world ocean, together with the equatorial Pacific and Southern Ocean. These regions are characterized by an excess pool of macronutrients delivered to the euphotic zone by mixing that is incompletely consumed during the annual biogeochemical cycle (Falkowski et al., 1998). The incomplete consumption has been explained by a lack of dissolved iron for which eolian dust is suggested to be the primary source in the SNP (e.g., Bishop et al., 2002, Duce and Tindale, 1991, Martin and Fitzwater, 1988, Martin et al., 1989, Rea, 1994). To test this hypothesis under modern conditions and in the past, we need reliable high-resolution dust records that are presently not available. The recent DIRTMAP3 compilation (Maher and Kohfeld, 2009, and references therein) with the addition of data based on surface water concentrations of dissolved aluminum (Measures et al., 2005) and 230Th-normalized 232Th fluxes (Kohfeld and Chase, 2011) show considerable variability in the Holocene spatial pattern of dust input to the SNP, varying between 0.1 and 21.4 g/m2/yr without a clear geographical pattern. Therefore, most studies rely on dust model predictions. Model reconstructions are challenged by the complexity of the topography and local climatic settings of the East Asian dust source regions which can result in over- or under-representations of important sources like the Taklimakan desert (Luo et al., 2003, Tanaka and Chiba, 2006, Yumimoto et al., 2009). Since arid regions in East Asia are proposed to be the dominant dust sources to the SNP (e.g., Duce et al., 1980, Husar et al., 2001, Sun et al., 2001, Tanaka and Chiba, 2006, Uno et al., 2011), with potential minor contributions from North Africa, Middle East and Central Asia (Creamean et al., 2013, Hsu et al., 2012, Tanaka and Chiba, 2006), their complexity can result in uncertainties of model predictions for the SNP.
In this study, we present results from an extensive set of multicorer core-top sediments covering the SNP. In combination with biogenic component and grain size distribution data as well as 230Th-normalized mass accumulation rates (MARs), we reconstruct spatial dust flux patterns from three unique and independent geochemical tracers of dust: 4He, 232Th and rare earth elements (REEs). We use geochemical data from dominant East Asian dust sources for long-range atmospheric transport (Ferrat et al., 2011, McGee, 2009) and from volcanic ash layers from ODP sites in the SNP (Cao et al., 1995a, Cao et al., 1995b) as lithogenic endmembers to determine the sedimentary dust component. We compare the proxy results with each other to verify their potential as dust flux proxies and compare our results to model estimates and published observational data.
Section snippets
Study area and material
Samples from the top one cm were used from 37 multicorer sediment cores recovered during the SO202-INOPEX cruise in 2009 (Fig. 1; Table 1; Gersonde, 2012). The Kamchatka Transect in the northwest SNP consists of stations 1–9, the Alaska Transect in the northeast SNP of stations 23–31 and the Southern Transect of stations 32–45 (Fig. 1). Sediment lithology varies over the cruise track from clay-bearing diatomaceous muds at stations 10–12 in the southern Bering Sea to foraminiferal sands at
Results
All INOPEX core-top data and dust source data are available in Supplementary Datasets 1 and 2 and through PANGAEA (doi.pangaea.de/10.1594/PANGAEA.823124).
Discussion
Determining the eolian dust component in near-land regions of the SNP is a geochemical challenge because a significant fraction of the lithogenic input may originate from other sources. A major source is volcanic ash from the surrounding Kurile, Aleutian, Kamchatka, Alaska and Japan volcanic arcs (Bailey, 1993, Jones et al., 1994, Jones et al., 2000, Maeda et al., 2007, Nakai et al., 1993, Olivarez et al., 1991, Otosaka et al., 2004, Shigemitsu et al., 2007, Weber et al., 1996). Also
Summary and conclusions
This study presents a core-top spatial survey of lithogenic input to the Subarctic North Pacific. We find contributions of eolian dust, IRD, volcanic ash, hemipelagic and riverine material to the lithogenic fraction of the sediments, requiring deconvolution of the different contributions to estimate the eolian dust flux. Grain size distributions and grain size data modeling indicate the presence of a component with an uniform fine grain size mode at ∼4 μm over the entire SNP; however, grain
Acknowledgments
We would like to thank Roseanne Schwartz, Marty Fleisher, Louise Bolge, Linda Baker, Pat Malone and Nichole Anest for laboratory support at LDEO. We are thankful to Youbin Sun and Ryuji Tada for providing dust source samples. The paper was improved by constructive comments by Ryuji Tada, Lucia Korff, 2 anonymous reviewers and the editor, Gideon Henderson.
This work was supported by U.S. National Science Foundation grant OCE1060907 to G.W. and R.F.A. INOPEX samples were obtained during the
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