Sources of Fe to the equatorial Pacific Ocean from the Holocene to Miocene

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Abstract

Biological productivity in the modern equatorial Pacific Ocean, a region with high nutrients and low chlorophyll, is currently limited by the micronutrient Fe. In order to test whether Fe was limiting in the past and to identify potential pathways of Fe delivery that could drive Fe fertilization (i.e., dust delivery from eolian inputs vs. Fe supplied by the Equatorial Undercurrent), we chemically isolated the terrigenous material from sediment along a cross-equatorial transect in the central equatorial Pacific at 140°W and at Ocean Drilling Program Site 850 in the eastern equatorial Pacific. We quantified the contribution from each potential Fe-bearing terrigenous source using a suite of chemical- and isotopic discrimination strategies as well as multivariate statistical techniques. We find that the distribution of the terrigenous sources (i.e., Asian loess, South American ash, Papua New Guinea, and ocean island basalt) varies through time, latitude, and climate. Regardless of which method is used to determine accumulation rate, there also is no relationship between flux of any particular Fe source and climate. Moreover, there is no connection between a particular Fe source or pathway (eolian vs. Undercurrent) to total productivity during the Last Glacial Maximum, Pleistocene glacial episodes, and the Miocene “Biogenic Bloom”. This would suggest an alternative process, such as an interoceanic reorganization of nutrient inventories, may be responsible for past changes in total export in the open ocean, rather than simply Fe supply from dust and/or Equatorial Undercurrent processes. Additionally, perhaps a change in Fe source or flux is related to a change in a particular component of the total productivity (e.g., the production of organic matter, calcium carbonate, or biogenic opal).

Introduction

The central equatorial Pacific (CEP) is a high nutrient low chlorophyll regime, where the availability of limiting micronutrients such as Fe has the potential to control the export of atmospheric CO2 to the deep sea via photosynthesis and the sinking of biological particles (the “biological pump”) (Martin et al., 1991, Martin et al., 1994, Murray et al., 1994, Coale et al., 1996, Coale et al., 1996, Fitzwater et al., 1996, Landry et al., 1997). While it is clear that the modern equatorial Pacific is Fe-limited, first-order questions remain regarding the sources of dissolved Fe to the region's surface waters in the past, to what degree the area may have been Fe-limited during critical time periods in Earth's climate history, and regarding the potential regulation of atmospheric CO2 exchange over glacial–interglacial cycles by the biological pump via Fe availability (Lefevre and Watson, 1999, Archer and Johnson, 2000, Jickells et al., 2005).

One leading hypothesis is that Fe is delivered to Pacific surface waters via eolian dust and that the extent of Fe-limitation may have decreased during glacial stages when global climate was drier and dustier, contributing to increased export production and thus a greater drawdown of atmospheric CO2 (Martin et al., 1991, Petit et al., 1999; and references therein). While high-latitude ice cores show increasing glacial dust concentrations (Petit et al., 1999), low-latitude marine sediment records from the equatorial Pacific do not show consistent relationships among climate state, bulk sedimentary Fe, terrigenous input, and/or inferred biological export production (see Rea, 1990, Rea, 1994, Murray et al., 1995, Anderson et al., 2006).

Alternatively, rather than being fertilized by wind-blown dust, the surface waters could instead, or also, be fertilized by upwelling of the Fe-bearing eastward flowing Equatorial Undercurrent (EUC). Because of the high relief, source rock composition, and high rainfall in Papua New Guinea (PNG), the region contributes a disproportionally large amount of terrigenous material into the global ocean (Milliman et al., 1999, Kineke et al., 2000). Dissolved chemical tracers have shown that the EUC inherits a chemical signature from the PNG region (Lacan and Jeandel, 2001, Mackey et al., 2002) that can be traced to the CEP as far eastward as 110°W (Landry et al., 1997; Coale et al., 1996, Coale et al., 1996, Gordon et al., 1997, Sholkovitz et al., 1999). As postulated by Wells et al. (1999), the tectonically-driven uplift and erosion of PNG volcanic material during the Late Miocene could have provided the necessary dissolved Fe to nourish the putative “Biogenic Bloom” inferred to have occurred between 4 and 7 Ma in the eastern equatorial Pacific (Farrell et al., 1995; plus others). Analogously, changes in sea level along the PNG continental shelf during Pleistocene glacial–interglacial cycles may also have impacted the erosion and subsequent delivery of Fe to the EUC, and such variability could be at the same frequency as the well-documented cycles in biogenic sediment accumulation in the CEP (Murray et al., 1993, Murray et al., 2000, and references therein).

Thus, we here determine the degree by which the CEP responded in the past to compositional variations in Fe supply. We chemically and isotopically characterize the source of terrigenous particulate material supplied to a meridional transect at 140°W for (a) the modern, and (b) glacial Marine Isotope Stages (MIS) 2, 6, 10, and 16, as well as (c) during the Biogenic Bloom documented from 4–7 Ma at Ocean Drilling Program (ODP) Site 850 (Fig. 1).

Section snippets

Tracing dissolved Fe sources via the terrigenous component

We assume that the source of the terrigenous particulate material is the same as that of dissolved Fe. We are forced to make this assumption because there is no method to quantify the amount or source of dissolved Fe in the surface water column at a given time in the past. However, because the modern distribution of particulate Fe–which is carried by the terrigenous component–to a first-order mimics the distribution of dissolved Fe throughout the water column (Fig. 2; Gordon et al., 1997), by

Sampling

The 140°W transect was sampled as part of the Equatorial Pacific Process Study of the Joint Global Ocean Flux Study (JGOFS) (Milliman, 1995, Milliman, 1996, Milliman, 1997). Surface samples were collected using a Multicorer, which gathers short tubes several ten's of cm deep along with the sediment/water interface. The five piston cores were selected because they provide records of export production through Pleistocene glacial–interglacial cycles. We abbreviate the full core denomination (e.g.,

Assessment of potential sources

To obtain an appropriate value to represent the composition of each source region that is likely to cover a large area and contain various rock types, we averaged multiple published data and used the median of each source region for the elemental ratios and multiple linear regression models. The chemical and isotopic compositional statistics used for Asian loess, South American eolian ash, Papua New Guinea, and Marquesas Islands (ocean island basalts; OIBs) are provided in Table 3. For PNG,

Chemical characterization and quantification of terrigenous (Fe) sources

Using chemical and statistical analyses, we have identified the sources of terrigenous material to be wind-blown upper continental crust (most likely loess originating from Asia), OIB from the nearby Marquesas Islands (Murray and Leinen, 1993, Murray and Leinen, 1996), volcanogenic ash (inferred to be from South America, as described further below), and island-arc debris (from PNG) (Fig. 3, Fig. 5). We further quantified the relative contributions of these potential sources by employing

Multiple linear regression modeling

In this section, we study variability in potential Fe source(s) as a function of climate state, regardless of changes in export production. Our first-order observation is that the modern meridional distribution patterns of elemental ratios and inferred relative source contributions closely parallel modern oceanographic and atmospheric processes (Fig. 3, Fig. 5). While discrimination diagrams of elemental ratios are helpful in quantifying the general trends of relative contributions between

Flux of specific terrigenous Fe sources

Here we examine whether there are changes in the flux, as opposed to composition, of the terrigenous- and Fe source(s) as a function of climate state. We are well aware of the on-going controversy regarding how to best calculate accumulate rates in this region, in the context of the strongly debated role–or lack thereof–of sediment focusing (e.g., Broecker, 2008, Winkler et al., 2005, Francois et al., 2007, Lyle et al., 2007; and references therein). However, as will be shown here, regardless

Relationship between source of terrigenous Fe and biological productivity

We here investigate whether there is a relationship between the source of Fe and biological export production. The quantification of export production in the paleoceanographic record is an on-going challenge. Various approaches utilize sedimentologic, micropaleontologic, chemical, and/or isotopic proxies (see multiple references in Murray et al. (2000) as well as more recent representative contributions by Eagle et al., 2003, Loubere et al., 2003, Pichat et al., 2004, Winkler et al., 2005; plus

Summary and conclusions

By identifying the sources of terrigenous material at given time periods of paleoceanographic interest in the central equatorial Pacific, we have inferred the potential sources and transport pathways of dissolved Fe to the region. The relative contributions of Asian loess, South American ash, eroded PNG material, and ocean island basalts are a function of both latitude and climate. Asian and South American eolian material represent the majority of the terrigenous component (accounting for more

Acknowledgements

This research used samples and data provided by JGOFS and ODP. ODP was sponsored by the NSF and participating countries under the management of Joint Oceanographic Institutions (JOI), Inc. Funding was provided by NSF grants OCE01-36855 and EAR02-33712 (RWM and TP) and 0137453 (SRH). We thank R. Anderson (LDEO) for providing the Th radioisotopic data, N. G. Pisias (Oregon State) for assistance with the multivariate statistical analyses, and L. Bolge (BU) for her analytical assistance. Thanks to

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