Nitrogen isotopic evidence for a poleward decrease in surface nitrate within the ice age Antarctic

Abstract

Surface sediment diatom-bound δ¹⁵N along a latitudinal transect of 170°W shows a previously unobserved increase to the South of the Antarctic Polar Front. The southward δ¹⁵N increase is best explained by the combination of two changes toward the South, a decrease in the isotope effect of nitrate assimilation (ε) and an increase in the degree of nitrate consumption, both associated with shoaling of the mixed layer into the seasonal ice zone (SIZ). New downcore records show high amplitude changes in diatom-bound δ¹⁵N during the last ice age, with intervals of higher δ¹⁵N, including the last glacial maximum, the transition between marine isotope stages 5 and 4, and marine isotope stage 6, while other intervals are similar in δ¹⁵N to interglacial sediments. Variation in the range of 0–3‰, as seen in previously published records, may be entirely due to changes in ε. However, the observed magnitude of the change of 4–10‰ in the three new records and the locations of these records relative to the modern meridional gradient in mixed layer depth appear to require increased nitrate consumption to explain the high-δ¹⁵N intervals. The new sites are near the modern Southern Antarctic Circumpolar Current Front (SACCF), and one of the sites has been shown to be associated with sporadic summer sea ice during the LGM. As with other Antarctic sites, the available proxy data suggest that they were characterized by lower export production. Based on these and other observations, we propose that the weak southward nitrate decrease in the modern Antarctic surface was a fully developed “nutrient front” in the glacial Antarctic, associated with the SACCF. Both modern ocean and paleoceanographic work is needed to test this hypothesis, which would have major implications for atmospheric CO₂.

Introduction

One of the primary features of the modern Antarctic Zone (AZ) in the Southern Ocean is the large excess of the major nutrients, nitrate and phosphate. At present, the nutrient-rich surface releases carbon dioxide (CO₂) that was previously sequestered in the ocean interior by the rain of biogenic debris out of the surface ocean in other regions. The rapid exposure of CO₂-rich water and incomplete consumption of nitrate and phosphate at the surface limits the degree to which the global biological pump is able to lower atmospheric CO₂. More complete utilization of nutrients during glacial episodes, either by increased productivity or reduced deep overturning, would have rendered the global biological pump more efficient by reducing the release of CO₂ from this region, potentially explaining the observation of lower atmospheric CO₂ concentrations during ice ages (Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984). Even a reduction in overturning without more complete nutrient utilization would have reduced atmospheric CO₂, albeit not as strongly (Toggweiler, 1999; Sigman and Haug, 2003).

The AZ, which surrounds the Antarctic continent, is one of three major zones of the Southern Ocean (Orsi et al., 1995) (Fig. 1). To the north, it is bounded by the Polar Frontal Zone (PFZ), which is separated from the Antarctic by the Antarctic Polar Front (APF) and from the more equatorward Subantarctic Zone (SAZ) by the Subantarctic Front. The AZ itself is divided into zones, the permanently open ocean zone (POOZ) and the seasonal ice zone (SIZ). The Southern Antarctic Circumpolar Current Front (SACCF) bounds the southern edge of the ACC and approximates the northernmost limit of the SIZ. The AZ is a region of large scale, wind-driven upwelling and the only region of the Southern Ocean that directly ventilates the abyssal ocean. As such, it plays a critical role in the air–sea balance of CO₂ (Marinov et al., 2006).

In the AZ, export production during the ice ages, as inferred from opal and biogenic Ba flux estimates, was lower than during interglacials (Charles et al., 1991; Mortlock et al., 1991; Kumar et al., 1993; Francois et al., 1997; Frank et al., 2000; Chase et al., 2003). Lower algal growth rates are also suggested by the lower ¹³C/¹²C of bulk and diatom-bound organic matter (Shemesh et al., 1993; Singer and Shemesh, 1995). The reduced productivity in the glacial Antarctic has been explained in two very different ways: as the result of light limitation, for instance, by increased sea ice cover shortening the growing season (Mitchell et al., 1991; Anderson et al., 1998, Anderson et al., 2002), or as the result of a decrease in the gross supply of nutrients from below (Francois et al., 1997).

For more than a decade, downcore N isotope studies have been conducted to test explanations for the observed productivity changes, by providing a constraint on the ratio of nitrate uptake to gross nitrate supply (i.e. on nitrate consumption). The ¹⁵N/¹⁴N of sinking organic matter reflects the utilization of nitrate (NO₃⁻) in regions such as the Southern Ocean where NO₃⁻ is not completely consumed (Altabet and Francois, 1994). Phytoplankton preferentially take up NO₃⁻ bearing the lighter N isotope, ¹⁴N. As the initial NO₃⁻ supply is progressively consumed, the δ¹⁵N of the NO₃⁻ increases, leading to a related increase in the δ¹⁵N of the organic matter produced from the NO₃⁻ (δ¹⁵N=((¹⁵N/¹⁴N)_sample/(¹⁵N/¹⁴N)_reference−1) where the N₂ in air is the universal reference). Early work focused on the use of bulk sedimentary δ¹⁵N despite the acknowledged potential for diagenetic artifacts due to alteration of the isotopic signal during sinking and sedimentation. Microfossil-bound organic N has since been targeted as a potentially pristine sedimentary N archive, and several measurement methodologies have been developed thus far (Shemesh et al., 1993; Sigman et al., 1999a; Robinson et al., 2004; DeLaRocha, 2006).

The balance of data for temporal changes in nutrient consumption within the AZ suggests that evidence for reduced export production in the glacial Antarctic was accompanied by a higher δ¹⁵N in sinking N, despite differences in measurement techniques and potential methodological problems (Francois et al., 1997; Sigman et al., 1999a; Crosta and Shemesh, 2002; Robinson et al., 2004). Beginning with Francois et al. (1997), workers have, on the basis of the N isotope results, inferred greater nitrate consumption during the last ice age. Coupling the evidence for more complete nitrate consumption with the evidence for a decrease in export production, they deduced a decrease in gross rate of supply of major nutrients to the Antarctic euphotic zone (Francois et al., 1997; Sigman et al., 1999a; Robinson et al., 2004; Sigman et al., 2004). Physically, this change would require reduced exchange of water between the surface and deep Antarctic, as would result from year-round stratification of the upper water column. However, the various types of δ¹⁵N data do not allow for a straightforward comparison of the different downcore records. Moreover, downcore records of diatom microfossil-bound δ¹⁵N measured with a single methodology (the persulfate-denitrifier method that is employed in this study) have suggested that the glacial decrease in export production was associated with varied degrees of enhanced nitrate consumption, spatially and temporally within the glacial AZ (Robinson et al., 2004).

Here, we report three new downcore profiles of diatom-bound N isotopes from the Atlantic and Pacific sectors of the Antarctic. The downcore diatom-bound δ¹⁵N records suggest large changes in the degree of nitrate consumption over the core sites in both sectors of the AZ. These data contrast with previously published profiles (generated with the same methods) and suggest broad spatial variations in nutrient consumption across the AZ. Surface sedimentary diatom-bound δ¹⁵N from a transect that spans the SIZ of the Antarctic, the PFZ and into the Subantarctic along 170°W provide some perspective on the new downcore results, suggesting the existence of a meridional gradient, albeit a modest one, in the modern AZ. Taken together, the downcore and modern δ¹⁵N data suggest that the southernmost region of the Antarctic has witnessed large-scale changes in nutrient consumption, closely associated with the presence of summertime sea ice. Since wintertime deepwater formation in the Antarctic currently occurs in the more polar Antarctic, this interpretation has important implications for atmospheric CO₂.