Here we present new proxy data from Ocean Drilling Program (ODP) site 1237 drilled on the easternmost flank of Nazca Ridge, 140 km offshore (16.11°S, 76.37°W; 3212m water depth), covering the past 500 ka. Its position below the path of eolian transport from the Atacama Desert and close to the arid coasts of Peru, but west of the deep-‐sea trench provides an excellent opportunity to reconstruct eolian input due to its undisturbed and complete sediment record (Mix et al., 2003). Our data represent the only continuous dust record from the southeast Pacific south of 5°S covering the last 500 ka. The approach of using multiple proxies including grain-‐size distributions, Th-‐isotopes, and the geochemical composition of the sediment allows us to differentiate between changes in wind intensities and climatic changes in the source areas (E.g. Rea et al., 1985; Olivarez et al., 1991). We compare our record to previously published dust flux data from the equatorial Pacific (Winckler et al., 2008) and to a multiproxy climate reconstruction at ODP Site 1239 located on Carnegie Ridge off Ecuador, constraining latitudinal shifts of the ITCZ on glacial-‐interglacial timescales (Rincón-‐Martínez et al., 2010).
7.2 Materials and Methods
The age model of site 1237 is based on a benthic δ18O record tied to the chronology of the LR04 isotope stratigraphy of Lisiecki and Raymo (2005), applying the Analyseries software (Paillard, 1996). Ba-‐counts of the XRF-‐scans were used for fine-‐tuning the age model (Appendix 3). The lack of sample material between 3.3 – 7.4 mcd (80 – 240 ka) at Site 1237 was compensated for by using samples from pre-‐site survey core RRV9702A-‐69PC (16.01°S, 76.33°W). The latter was aligned to ODP Site 1237 using magnetic susceptibility data from both cores (Appendix 3).
The siliciclastic content of ODP Site 1237 sediments represents the percentage of bulk sediment after subtracting measured percentages of biogenic opal, carbonate, and organic matter (TOC). The biogenic opal content (wt %) was determined with an automated leaching method, following procedures outlined in Müller and Schneider (1993). TOC and total carbon contents were analyzed with the LECO technique and a CNS elemental analyzer, respectively, before calculating CaCO3 contents, following standard methods (E.g. Rincón-‐Martínez et al., 2010). The chemical composition of the sediment was analyzed applying X-‐ray fluorescence scanning (XRF) (Aavatech 2nd generation XRF Scanner at the Alfred-‐Wegener-‐Institute, Bremerhaven (AWI)) and Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-‐
OES in the Geochemistry Department at AWI). We scanned the split sediment cores in 1 cm intervals corresponding to a time-‐resolution of 300 – 800 years. Iron XRF counts were calibrated to absolute concentrations determined by ICP-‐OES on 35 samples (cf. Rincón-‐
Martínez et al., 2010).
CHAPTER 7
Figure 7. 2 Records of eolian-‐derived sediment input to ODP site 1237 over the last 500 ka. (A) Benthic oxygen
isotope stack (Lisiecki and Raymo, 2005), (B) Benthic oxygen isotope record of ODP site 1237, (C) Linear sedimentation rates (LSR, cm ka-‐1), (D) siliciclastic accumulation rates (AR, g cm-‐2 ka-‐1), (E) Iron accumulation rates (Fe AR, mg cm-‐2 ka-‐1), (F) 232Thorium flux (µg cm-‐2 ka-‐1), (G) Mean grain size of siliciclastic sediment fraction (µm).
Late quaternary glacial-‐interglacial climate variability of western South America Inferred from eolian dust as preserved in marine sediments
Linear sedimentation rates (LSR, in cm ka-‐1) were derived from the age model and multiplied
We measured the grain-‐size distribution of the terrigenous siliciclastic sediment fraction with a Beckman-‐Coulter Laser Particle Sizer. To isolate the terrigenous sediment fraction, we during interglacials (see Appendix) to avoid dilution effects; we calculated accumulation rates (ARs) as the better proxy. This resulted in siliciclastic and iron (Fe) ARs that are increased by integrated over time intervals between age control points, assuming a constant accumulation rate over each interval, which, however, cannot be ascertained (Francois et al., 2004; Loubere the equatorial East Pacific comparing the different MAR-‐approaches, where one explanation could be sediment redistribution leading to focusing during glacials. Focusing factors at ODP Site 1237 average approximately 2 (not shown) and vary within a few tens of percent throughout the time interval in consideration. Sediment focusing could explain the observed 35 -‐ 65 % difference in the accumulation of bulk and constituent sediments at the study site
except of 3 outlying peaks >5.5µm, representing sediment core sections contaminated by volcanic ash. However, grain-‐size means do increase on a longer timescale, from 500 ka to 260 ka, when they drop to <4µm, only to slightly increase again from ~250 ka until ~30 ka, before another drop below 4µm at ~9 ka.
7.4 Discussion
7.4.1 Changes in dust flux on glacial-‐interglacial cycles
The data set presented in Figure 7.2 is the first record obtained from below the major dust-‐
transporting wind field in the southeast Pacific close enough to the source area to record changes in dust supply and wind intensity (Figure 7.1). Silt-‐sized material deposited in areas within a few hundred kilometers offshore is considered more reliable in recording these changes than the clay-‐sized fraction (E.g. Sarnthein et al., 1981; Tiedemann et al., 1989), which is the only fraction documented at the core locations of previous studies further offshore. The disadvantage of a study site as close to the coast as ODP Site 1237 is the hemipelagic sediment component one would usually expect, and which would contaminate the eolian input signal (E.g. Weltje and Prins, 2003; Weltje and Prins, 2007). However, we can rule out a hemipelagic component in the sediments of ODP cores 1237 as the site is located offshore, an area where no major, but only seasonal rivers drain into the Pacific. Furthermore, the South American deep-‐sea trench represents a barrier catching the fluvial material from the minor, ephemeral rivers of southern Peru, making winds the most probable transport agent of the terrigenous fraction (E.g. Krissek et al., 1980; Saukel et al., 2011).
Changes in MARs of dust provide a comprehensive picture of changes in the intensity of the dust cycle, including source, transport and deposition (Kohfeld and Tegen, 2007). Two different methods of calculating accumulation rates consistently show a 2-‐3-‐time increase of siliclastic material during glacials compared to interglacials at our site. Fe MARs calculated from LSRs and DBD show the same trend and amplitude of our 232Th-‐dust record (Figures 7.2e, f). Our records generally support the idea of dustier glacials compared to interglacials, in agreement with different modeling results for the last glacial maximum (LGM, E.g. Mahowald et al., 1999; Lunt and Valdes, 2002; Werner et al., 2002) and, in particular, support earlier results from the equatorial east Pacific (E.g. Winckler et al., 2008).
Late quaternary glacial-‐interglacial climate variability of western South America Inferred from eolian dust as preserved in marine sediments
Figure 7. 3 Comparison of eolian flux records in relation to SST records. (A) Benthic oxygen isotope stack (Lisiecki
and Raymo, 2005), (B) Dust flux of ODP Site 1237 (g m-‐2 a-‐1) (C) Dust flux of ODP site 849 (g m-‐2 a-‐1), B and C are calculated by dividing by the average 232Th concentration of upper continental crust, (D) Iron accumulation rates (Fe AR, mg cm-‐2 ka-‐1) of ODP Site 1237, (E) 230Th-‐normalized iron flux at ODP Site 1237, (F) Iron accumulation rates (Fe AR, mg cm-‐2 ka-‐1) of ODP Site 1239 (Rincón-‐Martínez et al., 2010), (G) Alkenone sea surface temperatures (SST, °C)
heavily influenced by the equatorial front, ODP Site 1239 (Rincón-‐Martínez et al., 2010), (H) Alkenone sea surface at all. Since the hemipelagic contribution to the terrigenous sediment component is negligible at our study site, the derived proxy record allows a re-‐interpretation of previously published results clearly oppose this hypothesis. According to Rincón-‐Martínez et al. (2010), terrigenous input during interglacials at ODP Site 1239 (Figures 7.1 and 7.3) was increased due to
Late quaternary glacial-‐interglacial climate variability of western South America Inferred from eolian dust as preserved in marine sediments