The transition on North America from the warm humid Pliocene to the glaciated Quaternary traced by eolian dust deposition at a benchmark North Atlantic Ocean drill site
Introduction
Deep-sea sediments in the climatically sensitive North Atlantic region are composed of two main constituents: biogenic carbonate (CaCO3) produced in the overlying water column and allochtonous detrital material, with volcanic ash important only locally. It has long been recognised that striking rhythmic changes in the abundance of these constituents and therefore sediment color and %CaCO3 (Fig. 1) provide both a high fidelity means of stratigraphic correlation and an expression of pronounced climate variability, especially in sediments deposited during times of significant northern hemisphere glaciation (NHG) (e.g. Ericson et al., 1961, Ruddiman and Glover, 1972).
Shackleton et al. (1984) drew attention to the remarkable correspondence between high amplitude changes in both benthic δ18O and %CaCO3 back to earliest Pleistocene Marine Isotope Stage (MIS) 100 (2.52 Ma) at Deep Sea Drilling Project (DSDP) Site 552, where sediment deposition is dominated by pelagic rain from above. Prior to 2.52 Ma, variance in benthic δ18O is unaccompanied by large amplitude change in %CaCO3 at Site 552 (Fig. 1A). Originally, initiation of high-amplitude variance in color and %CaCO3 at this site was attributed by Shackleton et al. (1984) to onset of major NHG with %CaCO3 controlled by variations in the flux of non-carbonate material transported by ice rafting. We now know that the exact timing of the large decrease in %CaCO3 in sediments deposited at Site 552 is obscured because MIS G6 through 103 (2.72–2.58 Ma) fall in a core break (Raymo et al., 1989). As it happens, however, extensive work elsewhere shows MIS 100 to be the oldest glacial during which ice sheets were large enough (Ruddiman et al., 1987, Jansen and Sjoholm, 1991, Maslin et al., 1998, Jansen et al., 2000, Kleiven–F et al., 2002, Bailey et al., 2013) and high latitude surface ocean temperatures cool enough (Lawrence et al., 2009, Lawrence et al., 2010, Naafs et al., 2010) to initiate ice-rafting on a basin-wide scale across the open North Atlantic Ocean.
In Fig. 1A we show %CaCO3 records from two further classic North Atlantic drill sites, DSDP 607 and 609 located on the southern fringe and at the centre of the last glacial ice-rafted debris, IRD, belt, respectively (Fig. 2). Originally, %CaCO3 variability at these two sites prior to ∼2.5 Ma was attributed to sea floor CaCO3 dissolution, a consequence of their greater water depth (Sites 609, ∼3.9 km & 607, ∼3.4 km, vs. 552–2.3 km, Fig. 1A) and the influence of corrosive poorly ventilated southern-sourced bottom waters (Ruddiman et al., 1987, Ruddiman and Raymo, 1988). Yet, comparison of the %CaCO3 plots compiled by Ruddiman et al. (1987) to records from shallower more recently drilled sites (Fig. 1A vs. 1B) reveals that the timing of the initiation of marked lithological cycles in North Atlantic Ocean sediments is not a simple function of water depth indicating the influence of some factor other than CaCO3 dissolution.
In principle, three mechanisms have the potential to deliver terrigenous sediments to Site 607. But negligible Pliocene rates of accumulation of sand-sized IRD and volcanic grains at Integrated Ocean Drilling Program (IODP) Site U1313 (Bolton et al., 2010b, Bolton et al., 2010a), the reoccupation of DSDP Site 607, confirms that the contemporaneous variability seen in %CaCO3 (Fig. 1) is not a function of melting icebergs over this classic site. Two alternative potential explanations must therefore be considered: (1) Transport beyond the contemporary iceberg front by ocean currents of fine-grained material delivered by ice-rafting to the Nordic Seas by the Greenland Ice Sheet (Winkler, 1999, Andrews, 2000, Jansen et al., 2000). (2) Transport of continentally derived eolian dust from North Africa or from North America as inferred based on biomarker records (Naafs et al., 2012).
To better understand the control(s) on, and climatic significance of, %CaCO3 variability of North Atlantic Ocean sediments deposited during the intensification of northern hemisphere glaciation (iNHG) we present new orbital-resolution records of carbonate dissolution, benthic δ13C, coarse lithic abundance and sediment %CaCO3 for IODP Site U1313, and radiogenic isotopes datasets that track the provenance of terrigenous inputs to this site. We show that lithological cycles in North Atlantic sediments of Pliocene age are driven by enhanced glacial fluxes of terrigenous material, not carbonate dissolution. Our provenance work indicates that the terrigenous component at the site is dominated by eolian dust sourced from the mid latitudes of North America – a result consistent with published interpretations of the record from Site U1313 of biomarkers derived from higher plant leaf waxes (Naafs et al., 2012). A sharp increase in the biomarker proxy for dust inputs to our study site during MIS G6, 2.72 Ma, is interpreted to reflect the importance of glacial grinding by a large North American ice sheet complex in amplifying dust inputs to the North Atlantic Ocean during glacials from this time (Naafs et al., 2012). Comparison among data sets, however, indicates strong non-linearity in coupling between the dust and biomarker records indicating that a reappraisal is merited of the sequence of climatic events that they record and the mechanisms involved.
Section snippets
IODP Site U1313
IODP Site U1313 is located at the base of the upper western flank of the Mid Atlantic Ridge at a water depth of 3426 m, ∼240 nautical miles northwest of the Azores archipelago (41°N, 32.5°W), on the extreme southerly limit of the last glacial “IRD belt” (Ruddiman, 1977), a southwest-northeast trending band of maximum iceberg melting and hence IRD deposition between ∼40°N and 55°N in the Atlantic Ocean (Fig. 2). Site U1313 was drilled during IODP Expedition 306 and constitutes a reoccupation of
Stable isotope stratigraphy and sediment color
The record of benthic δ13C at Site U1313 shows only modest glacial–interglacial variability with the exception of prominent excursions to low values during the large benthic δ18O glacials MIS 100, 98 and 96 (Fig. 5). This result is consistent with the record from predecessor Site 607 (Raymo et al., 1989), but the prominent (inter)glacial δ13C signal established in MIS 100 is more pronounced in our record. Our record also resolves with higher fidelity earlier key glacials and illustrates, for
Conclusions
We present Plio-Pleistocene records of sediment color, %CaCO3, foraminifer fragmentation, benthic δ13C, coarse lithic counts and the radiogenic isotope (Nd, Sr, Pb) composition of terrigenous sediment component from IODP Site U1313. We demonstrate that glacial–interglacial cycles in sediment color are unambiguously correlated to benthic δ18O back to at least 3.3 Ma, and represent changes in sediment %CaCO3. Our new records of terrigenous sediment accumulation rates, foraminifera fragmentation
Acknowledgements
This research used samples provided by IODP, which is sponsored by the US National Science Foundation and participating countries under management of Joint Oceanographic Institutions, Inc. We thank the shipboard party of IODP Expedition 306. We also thank W. Hale and A. Wuelbers for help with sampling and D. Spanner, M. Bolshaw and A. Michalik for laboratory assistance. We are grateful to the editor, C. Hillaire-Marcel, two anonymous reviewers and Stijn De Schepper for detailed constructive
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- 1
Present address: Camborne School of Mines, College of Engineering, Mathematics & Physical Sciences, University of Exeter, Penryn Campus, Treliever Road, Penryn, Cornwall TR10 9FE, UK.
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Present address: Facultad de Geología, Universidad de Oviedo, Campus de Llamaquique, Jesús Arias de Velasco s/n, 33005 Oviedo, Spain.
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Present address: Institute of Earth Sciences, University of Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany.
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Present address: Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK.
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Present address: GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany.