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Elevated Foraminifera-bound Nitrogen Isotopic Composition During the Last Ice
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Age in the South China Sea and Its Global and Regional Implications:
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Auxiliary Materials
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Auxiliary method:
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The total N (TN), organic N (Norg) and organic carbon (Corg) contents as well as the
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isotopic composition (δ15N and δ13Corg) were analyzed at site MD9702142, using an
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elemental analyzer (NC2500 Carlo Erba) coupled with a Thermo Finnigan Deltaplus
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Advantage isotope ratio mass spectrometer (IRMS) at the Academia Sinica, Taiwan.
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Samples were treated with 1N HCl for 16 h to remove carbonate; the residue was
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centrifuged and freeze-dried, following protocols in Kao et al., [2006]. USGS 40, which
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has certified δ13C of -26.24‰ and δ15N of -4.52‰, and acetanilide (Merck) with δ13C of -
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29.76‰ and δ15N of -1.52‰ were used as working standards. The reproducibility of
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carbon and nitrogen isotopic measurements is better than 0.15‰.
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Auxiliary results and discussions:
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δ15N difference among the euphotic zone dwelling species
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In the ODP Site 999 FB-δ15N record from the Caribbean Sea, the δ15N
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relationships among G. ruber, G. sacculifer, and O. universa change from the last ice age
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to the current interglacial (Main text Fig. 4c). Whereas the δ15N of the three species were
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indistinguishable during the deglaciation and Holocene, they were clearly different
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during the last ice age, with the mixed layer dweller G. ruber the lowest and the deep
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chlorophyll maximum dweller O. universa the highest. This was interpreted as indicating
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a change in the importance of N fixation relative to subsurface nitrate supply, with a
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reduction in N fixation during the last ice age leading to a stronger vertical gradient in the
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particulate N food source to foraminifera within the euphotic zone [Ren et al., 2009]. In
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the SCS record, the δ15N difference among the three species is on average smaller during
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the current interglacial than during the last ice age, and the maximal difference occurs at
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the LGM (Fig. S2b), in the same sense as the data from the Caribbean. The similarity of
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the three species in late Holocene sediment is consistent with the lack of depth gradient in
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the δ15N of the suspended POM in the 50 m euphotic zone of the modern SCS [Loike et
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al., 2007; Yang, unpublished data]. A similar interpretation for the LGM differences as
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put forward for the Caribbean data would thus seem to apply to the SCS.
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However, the SCS data show much smaller inter-species changes than observed in
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the Caribbean LGM sediments; in particular, G. sacculifer and O. universa show no clear
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δ15N difference throughout the SCS record. Even with the data smoothed (Fig. S2), the
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evidence for a greater difference between G. ruber and the other species during the LGM
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is subtle. Modern ocean studies are needed to truly address this question. Nevertheless,
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we suspect that the more muted signal in the SCS derives from the depth compression of
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features and the weaker stratification and less clear vertical structure within euphotic
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zone of the central to northern SCS than in the Caribbean. In the SCS, upwelling and
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other effects from the monsoons bring micromolar levels of nitrate to within 50 m of the
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surface, such that the SCS lacks a nitracline and deep chlorophyll maximum that is
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clearly segregated from the wind-mixed layer [Liu et al., 2002; Wong et al., 2002, 2007].
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It is thus very likely that the depths of maximum occurrence of the three euphotic zone-
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dwelling foraminifera have always been much more similar than in the Caribbean, with a
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parallel convergence in the biogeochemical conditions that they experience. If so, the
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isotopic divergence of the euphotic zone species during the last ice age would be
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expected to be weaker than in the Caribbean, as observed.
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Bulk sedimentary measurements
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Previously published bulk sedimentary δ15N records from the SCS show minimal
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glacial/interglacial difference and were interpreted as evidence for a constant mean ocean
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nitrate δ15N [Kienast, 2000]. However, our FB-δ15N record is drastically different from
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the bulk sedimentary δ15N record at this site (Fig. S3) and from previous studies in SCS
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[Higginson et al., 2003; Kienast, 2005], questioning the reliability of bulk sedimentary
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δ15N in the SCS. The use of bulk sedimentary δ15N as a recorder of past changes in the
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δ15N of N export may be compromised by contamination from terrestrial or shelf N input,
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as well as by bacterially driven degradation, which can significantly elevate the δ15N of
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sedimentary N relative to the N sinking out of the surface ocean [Altabet and Francois,
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1994; Lourey et al., 2003]. The sharp rise in the bulk sedimentary δ15N near the coretop
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at MD97-2142 is likely due to preferential loss of light nitrogen isotope during
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degradations. Records of carbonate, organic carbon content, carbon to nitrogen ratio and
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δ13C of the organic matter at MD97-2142 show long- and short-term variations that may
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offer insights into the bulk sedimentary δ15N record and its differences from the FB-δ15N
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record.
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Consistent with previous studies at this and other sites in SCS, the glacial section
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at MD97-2142 has lower carbonate and higher total organic carbon (TOC) content
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relative to the current interglacial (Fig. S3a,b), which may be due to greater terrigenous
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input during times of low sea-level stand [Chen et al., 2003; Shiau et al., 2008] as well as
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higher productivity in the last ice age [Lowemark et al., 2009]. Although the δ13C record
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suggests a marine origin for most of the TOC, higher C/N ratio during the last ice age
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indicates a greater contribution of terrestrial inputs (Fig. S3c), which is also supported by
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evidence of higher terrigenous organic biomarker concentrations during the last ice age in
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the SCS [Pelejero, 2003]. Today, the bulk sedimentary δ15N of the nearest river
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discharge, the Pasig River from the Luzon Island, is around 2 to 3 ‰, which significantly
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lowers the δ15N of the surface sediment in the basin near the river [Gaye et al., 2009].
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Thus, we suspect that the glacial bulk sedimentary δ15N could have been lowered by
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greater terrestrial inputs.
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More relevant to FB-δ15N than bulk δ15N, TOC content shows an increase starting
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at ~25 ka, which is possibly related to the early rise in FB-δ15N (Fig. S3). There appears
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to be a deglacial input of terrigenous material (see decline in δ13C) that lowers TOC and
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carbonate content. If these decreases in TOC were stripped from the records, then the
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TOC record would show a broad maximum centered on roughly ~13 ka. This TOC
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record has been interpreted as indicative of changing SCS productivity [Chen et al.,
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2003; Lowemark et al., 2009]. If so, this would be consistent with our suggestion that a
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monsoon-driven shoaling of the thermocline occurred late in the last ice age and waned in
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the mid-Holocene; we called upon this above as a possible explanation for the late glacial
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increase in FB-δ15N and then the mid-to-late Holocene decrease in FB-δ15N,
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superimposed on the deglacial ocean nitrate δ15N maximum and the glacial-to-interglacial
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increase in N fixation (Main text Fig. 5). Above, we also noted an 8.3 ka maximum in
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FB-δ15N that coincides with a faunal and G. ruber δ18O event, all of which could be
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explained by a weakening of the thermocline, perhaps driven by stronger upwelling. We
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note here that this 8.3 ka event overlaps with a sedimentological event (peaks in TOC and
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carbonate) that is also consistent with a brief sharp shoaling of the SCS thermocline, in
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that such a shoaling would increase productivity. However, we must recognize that this
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view of the bulk sediment data is selective; for example, there is also an earlier
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sedimentological event (at ~10.5 ka) for which there is no corresponding feature in the
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FB-δ15N record.
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Auxiliary figure captions:
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Figure S1
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Locations of the study site MD97-2142 (solid symbol, 12°41’N, 119°27’E, water depth
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1557m), and two comparison sites from previous studies (open symbols), MD012404
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from Okinawa Trough (26°39’N, 125°48’E, water depth 1397m, Kao et al., 2008), and
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MD98-2181 from west equatorial Pacific (6°18’N, 125°49’E, water depth 2114m,
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Kienast et al., 2008).
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Figure S2
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a) Three-point running average of foraminifera-bound δ15N. Blue: O. universa; Red: G.
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sacculifer; Green: G. ruber. b) The difference between δ15N of G. ruber and the average
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δ15N of G. sacculifer and O. universa. c) δ18O of G. ruber (white) calcite (‰ v. VPDB)
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from Wei et al., 2003.
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Figure S3
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Relevant additional measurements from sediment core MD97-2142 from the South China
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Sea. a) TOC content (black filled circles) in comparison with June insolation at 30°N
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(blue curve, [Berger and Loutre, 1991]). b) Carbonate contents (red filled circles). c)
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Atomic ratio of carbon and nitrogen (red open circles) and δ13C of organic matter (black
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open circles). d) δ15N of bulk sedimentary nitrogen in black crosses. e. δ18O of G. ruber
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(white) calcite (‰ v. VPDB) [Wei et al., 2003]. The purple arrows highlight two events
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that appear to be characterized by colder, more vertically mixed conditions and higher
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surface productivity in the SCS. The second of these events (but not the first) corresponds
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to an FB-δ15N maximum for all three euphotic zone dwelling species (Main text Fig. 6).
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Auxiliary references:
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