9.1. Summary and conclusions
9.1.2. Late Pleistocene source and flux of terrigenous sediments into the eastern equatorial Pacific
The second part of this thesis comprises three case studies (chapters 5, 6, 7) focused on the transport and deposition of terrigenous material in the easternmost Pacific Ocean at different time periods in the Late Pleistocene. These studies used a multi-‐proxy approach combining inorganic (elements and element ratios) and organic (biomarker) proxies, as well as bulk geochemistry and grain-‐size analysis. Fluvial and eolian transports are the most important pathways for the transference of terrigenous material into the ocean. One case study therefore deals with the long-‐term variability in the accumulation of fine-‐grained sediments that reach the deep sea off the Ecuadorian coast (ODP Site 1239; Chapter 5), while other study investigates on long-‐term eolian dust deposition in the deep sea off the Peruvian coast (ODP Site 1237; Chapter 7). Besides tracing terrigenous input into these deep-‐sea environments, the Lima Basin case study (Chapter 6) also reconstructed paleo-‐conditions on the continent based on specific biomarkers.
Based on the geographical location and oceanographic/atmospheric settings of each site we hypothesized that for ODP Site 1239, drilled close to the eastern crest of Carnegie Ridge (Sections 2.4.1 and 3.1.2; Figure 2.4b), terrigenous sediment supply comes from the mainland delta systems formed around river mouths located along the continental shelf (I.e. Guayas and/or Esmeraldas drainage systems). Although these rivers deposit most of their sediments in the Ecuadorian Trench, the eastern portion of the Carnegie Ridge still receiving a moderate load of continental detritus (Figure 2.13; E.g. Pazmiño, 2005). Additionally, based on 232Th flux measurementsSingh et al. (2011) established that in the Panama Basin, included the Carnegie Ridge,the location at which eolian (as opposed to riverine) fluxes dominate the detrital flux occurs at approximately 300 km from the margin. On the other hand, ODP Site 1237, drilled on the easternmost flank of Nazca Ridge, lies below the wind-‐driven surface currents of the southeast Pacific (Sections 2.4.1 and 3.1.3; Figure 3.2), therefore recording mainly deposition of terrigenous eolian components from the Atacama Desert and the arid coasts of Peru (E.g.
Molina-‐Cruz and Price, 1977; Scheidegger and Krissek, 1982).
Table 9.1. Selection of published Ti/Al and Fe/Al ratios, taken from Plewa et al. (2012).
The Al/Ti, Ti/Al and Fe/Al ratios can be used to further constrain the potential source of the terrigenous material, since different rock types have different ratios (Table 9.1). Regarding the Al/Ti ratio ODP Site 1239 exhibit values in a range from 11 to 25 (mean 17.53 ± 2.78) with no glacial-‐interglacial pattern, while ODP Site 1237 is slightly higher (mean 20.42 ± 2.51), varying CHAPTER 9
between 15 and 30. According to Pye (1987), the Al/Ti ratio of windblown material is 21. This points to dust as the most probable terrigenous source at ODP Site 1237. For comparison, the Al/Ti ratio of average continental crust is 15.6, average upper crust is 26.8, granites are even 40 but basalt or oceanic crust are lower than 10 (Taylor and McLennan, 1985). The Fe/Al ratio (0.49 ± 0.07 for ODP Site 1239 and 0.50 ± 0.05 for ODP Site 1237) does not show any significant downcore variations. Hence, by comparing the values reported in Table 9.1 with those from our study sites, the terrigenous matter for ODP Site 1239 (Ti/Al 0.06 ± 0.01) seems to come mainly from river suspended matter, while for ODP Site 1237 (Ti/Al 0.05 ± 0.01) the terrigenous matter could be mainly derived from windblown dust. Besides confirming our initial hypothesis about the source for terrigenous matter, both ratios (Ti/Al and Fe/Al) also validate that the terrigenous source did not change over the last 500 kyr in any of the studied cores.
Figure 9. 2 Map showing location of studied cores in the Panama Basin, from Singh et al. (2011). Red circles
represent cores analyzed by Singh et al. (2011) and yellow circles represent cores studied by others (Kienast et al., 2007; Loubere et al., 2001). Focusing factors are bracketed next to each core identification (first number in bracket represents Holocene (0–13 ka) focusing factor and second number represents glacial (13–25 ka) focusing factor.
The content and accumulation rates of siliciclastic material, iron (Fe), titanium (Ti), and lipid biomarker taraxerol at ODP Sites 1239 and 1237 exhibit a consistent glacial/interglacial pattern over the past 500 kyr. In the former, sediments are predominantly terrigenous during interglacials, while glacial siliciclastic supplies are substantially lower (Figures 5.3). Further support is provided by the 232Th fluxes calculated by Singh et al. (2011), which demonstrate that at cores V19-‐27 (close to ODP Site 1239), ME0005-‐27JC and TR163-‐38, which are closest to the South American margin, fluxes are higher during the Holocene than those during glacial (Table 9.2). Further to the west 232Th fluxes of non-‐margin cores are higher in glacial than in the Holocene, implying that as one moves away from the continent, the detrital signal becomes predominantly eolian derived (Singh et al., 2011).
Glacial-‐interglacial siliciclastic AR variations at Site 1239 (Figure 5.3) are unlikely due to post-‐
depositional processes such as horizontal focusing or winnowing. This part of the Carnegie Ridge is thickly clearly depositional and probably has not supplied much additional sediment to the Panama Basin (Singh et al., 2011). In seismic survey for drilling on ODP Leg 202 (line 6;
Shipboard Scientific Party, 2003a) it is possible to estimate how variable the average sedimentation rates have been based on the depth to the first major seismic horizon compared to its depth at Site 1239. Site 1239 has a sedimentation rate of 4.8 cm/kyr in the upper 50 m. Along line 6, 60% of the profile has a sedimentation rate between 0.5 and 1.5X—
that of Site 1239, 15% of the profile has rates >1.5X— that of Site 1239, and a little less than 20% of the profile has sedimentation rates ~0.5X— that of Site 1239 (Singh et al., 2011). Based on this and inspection of the other seismic lines, Singh et al. (2011) concluded that the sedimentation rate on the ridge appears to be uniform. Moreover, for margin cores V19-‐27 and TR163-‐38, located on the easternmost Carnegie Ridge, the average focusing factors during the Holocene and glacial are 1 and 1, respectively (Figure 9.2), so glacial focusing factors imply that sediment has not been redistributed at the studied site. Further south, cores TR163-‐33 and ME0005A-‐27JC exhibit 1.4 and 1.8 during Holocene, respectively (Figure 9.2), indicating that sediment in excess of what has been delivered vertically has been advected by deep-‐sea horizontal advection (I.e. focusing) to the studied site.
Table 9.2. Spatio-‐temporal variability of 232Th flux in the Panama Basin, from Singh et al. (2011). 232Th flux data for the first five cores are from Kienast et al. (2007), while the remaining data are from Singh et al. (2011).
Two different methods of calculating accumulation rates at ODP Site 1237 consistently show increased siliciclastic and Fe accumulation rates by factors of 2-‐3 during glacials compared to interglacials (Figure 7.2). Dust flux analyses based on 232Th data also suggest that there were two-‐fold increases in eolian fluxes during glacial in the central equatorial (E.g. Anderson et al., 2006; McGee et al., 2007; Winckler et al., 2008) and eastern equatorial Pacific Ocean (E.g.
McGee et al., 2007; Winckler et al., 2008). Among those studies, the easternmost cores are located at 110°W, and the dust source is hypothesized to change from northern-‐sourced (Chinese and North American) to southern-‐sourced (Africa, Australia and South American) dust between 5°N and 0°N in the equatorial Pacific (McGee et al., 2007). Although our study provides the record of the core furthest to the east (76.37°W) and closest to the South American continent, magnitude of the AR variations is very similar to the Glacial/ Holocene variations at both 110°W (McGee et al., 2007; Winckler et al., 2008) and 140°W (Anderson et al., 2006). Dust fluxes at Site 1237 are >30 times those measured in cores located on the equator at 110°W, which we explained by the proximal position of core 1237 to the continent.
Given the large distance between the cores, the coherence of the dust flux records between the two sites is surprisingly good, supporting the hypothesis of a common source. It is clear that the major part of the detrital fractions at these two locations must be eolian.
Focusing factor at ODP Site 1237 average approximately 2, indicating that sediment in excess of what has been delivered vertically has been advected by deep-‐sea horizontal advection (I.e.
CHAPTER 9
focusing) throughout the time interval in consideration. Sediment focusing could explain the Pacific, revealed by alkenone-‐derived glacial-‐interglacial SST amplitudes of up to 3.5°C (Figure 5.5.e). Conversely, during glacials, the W-‐E gradient suggests an intensified Walker circulation Section 3.1.5). Systematic glacial-‐interglacial patterns are also recorded in the meridional SST gradient record (Figures 5.5b and c). Low meridional SST gradients occur during interglacials, while glacials were characterized by a steeper SST gradient, with SST amplitudes of up to 2.6°C.
Meridional SST gradients within the cold tongue (ODP Site 1239 – TG7, off southern Peru) change in a glacial-‐interglacial pacing, the gradient being 4 -‐ 5°C during interglacials and 6 -‐ 7°C during glacials (Figures 7.3g-‐h). Then, the intensity of equatorward winds, a part of the Hadley atmospheric circulation, was likely increased during glacials. In the modern climate system this intensificationis accompanied of displacements of high-‐pressure cells closer to the continental low, and equatorial upwelling activity (see sections 1.3 and 2.2). The opposite settings are