Authigenic Nd isotope record of North Pacific Intermediate Water formation and boundary exchange on the Bering Slope
Introduction
The formation of intermediate and deep water plays an important role in the global climate system as a driving force of global ocean circulation and as a regulator of greenhouse gases such as carbon dioxide (CO2) (Biastoch et al., 2008, Häkkinen and Rhines, 2004, Stouffer et al., 2006). The North Atlantic Deep Water (NADW) is one such example, and past variations in its strength are well documented, e.g., the reduction in NADW formation and related climate change during the Younger Dryas (YD) and the Heinrich events (HE) (Sarnthein et al., 1994, Seidov and Maslin, 1999). There is no deep-water formation in the North Pacific at present due to the low surface water salinity. Intermediate water forms in the Okhotsk Sea through brine rejection, but the magnitude is relatively small compared to the NADW (Shcherbina et al., 2003, Talley, 1991, Yasuda, 1997).
Recent research shows that the formation of the North Pacific Intermediate Water (NPIW) was stronger during glacial periods and that the Bering Sea was a probable location (Horikawa et al., 2010, Ohkushi et al., 2003, Rella et al., 2012). For example, Ohkushi et al. (2003) propose that NPIW formed in the Bering Sea during the Last Glacial Maximum (LGM), based on the spatial distribution pattern of radiolarian species Cycladophora davisiana, which lives in a cold and well-oxygenated habitat such as the present NPIW environment. Supporting evidence comes from the δ18O of benthic foraminifera (δ18Obf) on the Bering Slope (MR06-04-23, 60°09.52′N, 179°27.82′W; 1002 m water depth) (Rella et al., 2012) (Fig. 1). There is a positive δ18Obf excursion during the LGM that cannot be interpreted by an increase in global ice volume alone but requires the intrusion of colder and/or more saline water to the site (Rella et al., 2012). Rella et al. (2012) suggest that NPIW formed during other cold periods as well, such as HE 1 and 4 and YD, based on the positive δ18Obf gradient between the Bering Slope and the Okhotsk Sea. Horikawa et al. (2010) use a different proxy to arrive at similar conclusions. The neodymium (Nd) isotopic composition of the intermediate water on the Bowers Ridge in the Bering Sea (BOW-8A, 54°47′N, 176°55′E, 884 m water depth) over ∼150 kyr shows radiogenic peaks during glacial periods, which they interpret as periods of NPIW formation (Horikawa et al., 2010) (Fig. 1, Fig. 2).
The premise of the NPIW-NADW seesaw hypothesis is that when NADW formation is diminished, NPIW formation regulates the global climate system by driving global ocean circulation and by distributing heat from equator to pole (Okazaki et al., 2010). Accordingly, periods of strong NPIW should correlate with those of weak NADW. Several model simulations suggest a seesaw relationship between NPIW/NPDW and NADW (Hu et al., 2012a, Okazaki et al., 2010, Saenko et al., 2004), but proxy studies are controversial. One proxy, the 14C ventilation age, is determined by the difference in 14C age between coexisting benthic and planktonic foraminifera. A low 14C ventilation age indicates downwelling or upwelling. Okazaki et al. (2010) report an anti-phase relationship in 14C ventilation ages between NPIW/NPDW (1000–2700 m water depth) and NADW during HE 1. Max et al. (2014) also show an anti-phase relationship between NPIW (700–1750 m water depth) and NADW during HE 1 and YD, but they argue against the seesaw with regard to NPDW (2100–2700 m water depth). An in-phase correlation between NPDW (2300 m water depth) and NADW during HE 1 is documented by Sarnthein et al. (2013). Another proxy is the δ13Cbf difference between two sites (Δδ13Cbf), which can be used as a proxy for NPIW or NADW formation (Knudson and Ravelo, 2015). A cross-spectral analysis of Δδ13Cbf over 1.2 Myr does not exhibit an anti-correlation between NPIW (818 m) and NADW, and it challenges the orbital-scale “seesaw” behavior (Knudson and Ravelo, 2015). This disagreement among proxies may be due to several factors. First, the uncertainty in the carbon reservoir age leads the authors to propose incompatible NPIW/NPDW behavior. For example, Okazaki et al. (2010) applied a smaller reservoir age (ΔR = 100 yr) than Max et al. (2014) (ΔR = 900 yr), which led them to propose NPDW formation during the cold HE 1 (Okazaki et al., 2010) and during the warm Bølling/Allerød (Max et al., 2014). Second, water mass-dependent variation in δ13Cbf can be smoothed out by biological productivity, air-sea gas exchange and/or calcite preservation. Third, the seesaw behavior may be episodic, e.g., only when freshwater discharge interrupts NADW formation like during HE 1. Therefore, a sufficient data-set covering multiple glacial-interglacial cycles with different proxies is necessary.
The Nd isotope ratio is a quasi-conservative water mass tracer (Frank et al., 2002, Rutberg et al., 2000). 143Nd is the daughter isotope of 147Sm, and the 143Nd/144Nd ratio in rocks is primarily determined by their Sm/Nd ratio and age. The Nd isotopic composition is reported as εNd, where εNd = [(143Nd/144Nd) sample/(143Nd/144Nd) CHUR − 1] × 104 and (143Nd/144Nd) CHUR is 0.512638 (Jacobsen and Wasserburg, 1980). The εNd values of local rocks influence those of adjacent sea water, as indicated by the similarity of εNd values between the ocean and the surrounding continents (e.g., Arsouze et al., 2007, Lacan and Jeandel, 2005). For example, the εNd values of the Pacific Ocean (εNd ∼0 to +4) surrounded by young volcanic rocks are more radiogenic than the Atlantic Ocean (εNd ∼ -13) surrounded by old continental rocks, and εNd in the Circumpolar Deep Water (CDW) is intermediate, reflecting mixing of the two water masses (εNd ∼ −9 to −6) (data from Albarède and Goldstein, 1992 and references therein; Albarède et al., 1997). This pattern attests to the residence time of Nd shorter than the ocean mixing time (Tachikawa et al., 1999) and to the utility of Nd isotopes as a quasi-conservative water mass tracer. The good correlation between εNd and the salinity of seawater supports the effectiveness of this proxy (von Blanckenburg, 1999).
The bottom water εNd is regulated by external and internal sources. External Nd sources are lateral advection (e.g. mixing between NADW and CDW; Rutberg et al., 2000) and vertical mixing (e.g. NADW formation) of water masses, and internal sources are boundary exchange (e.g., Arsouze et al., 2007) and pore water diffusion (e.g., Abbott et al., 2015). In the Atlantic, water circulation at present is relatively fast and vigorous due to NADW formation, and the effectiveness of internal sources is reduced. Thus, εNd can be used as a water mass tracer (Siddall et al., 2008). In the Pacific, water circulation is more sluggish (Siddall et al., 2008), and internal sources need to be taken into account. Indeed, ɛNd change in Pacific deep waters at present cannot be interpreted solely by water mass mixing (Abbott et al., 2016). Likewise, negligible NPIW formation in the Bering Sea at present (Warner and Roden, 1995) implies that bottom water ɛNd is mainly determined by internal sources. However, stronger NPIW formation in the Bering Sea in the past may switch the main Nd source from internal to the external, and provide a perspective on the interplay between external and internal sources controlling bottom water ɛNd.
We use the authigenic Fe-Mn oxyhydroxide coating of marine sediments as an archive of seawater Nd isotopic composition. Dissolved Nd is incorporated into the authigenic coating when it precipitates. Because this Fe-Mn oxyhydroxide coating is extracted from bulk sediments using chemical reagents, the dissolution of labile non-authigenic material (e.g., volcanic material) during the leaching process may bias the Nd isotope ratios (Roberts et al., 2010). The potential incorporation of non-authigenic components is tested by comparing the εNd of bulk sediment leachate with that of biogenic material such as foraminifera or coral reef (Martin and Scher, 2004, Roberts et al., 2010, van de Flierdt et al., 2010). In these tests, elemental concentration criteria (e.g., Al/Nd and REE pattern) are used to verify the authenticity of the leachates (Blaser et al., 2016, Martin et al., 2010, Wilson et al., 2013).
Here, we provide orbital-scale variations in NPIW at 1008 m using the isotope ratio of seawater-originated Nd on the Bering Slope, the most probable location of NPIW formation within the Bering Sea, as inferred from the distribution pattern of the C. davisiana during the LGM (Ohkushi et al., 2003). To acquire an authentic seawater signal in the Bering Sea, we first compare the Nd isotope ratio and elemental concentrations derived from three conventional extraction methods and determine the most appropriate method for Bering Slope sediments. With the Nd isotope record, we can constrain whether NPIW formation or boundary exchange processes control the bottom water ɛNd on the Bering Slope. Then, we examine the relationships between NPIW formation and cold climate and between NPIW and NADW.
Section snippets
Sampling location
Sediment core samples are from site U1345 (60°9′N, 179°28′W; 1008 m water depth), drilled on the western part of the Bering Slope during the Integrated Ocean Drilling Program Expedition 323 (Fig. 1). Site U1345 is influenced by tidal mixing and the Bering Slope Current (BSC) that flows westward along the continental shelf. This water circulation contributes to the high nutrient concentration and high biological productivity at this site (Springer et al., 1996), and the high sedimentation rate
Authigenic Fe-Mn oxyhydroxide fraction
The authigenic Fe-Mn oxyhydroxide fraction records past seawater composition and is therefore our main interest. During the extraction process, it is very important to avoid the dissolution of non-authigenic material. The dissolution of terrigenous components mainly depends on lithology; therefore, the chemical protocol needs to be adjusted to the specific site (Wilson et al., 2013). To determine the most suitable extraction method for the Bering Slope sediments, we first carried out a test on
εNd and elemental composition results
The Nd isotopic composition of the six different fractions of the four test samples is shown in Fig. 3A, B, K and L. When the extraction test was repeated, the εNd of each fraction was reproduced within uncertainty (0.2–0.5 εNd) (Fig. 3B). The detrital fraction had different εNd in the duplicate extractions, probably due to sediment heterogeneity. Of the six different fractions, the detrital fraction has the lowest εNd value. Among the non-detrital fractions, the carbonate fraction and Wilson
Conclusion
We reconstructed a 520-kyr record of the neodymium isotope ratio of the bottom water on the Bering Slope using the bulk sediment leachates. To establish the reliability of the data, we examined six different fractions for their Nd isotopic composition and elemental concentrations, such as Al, Fe, Mn, Sr, and REEs. The six fractions were the carbonate fraction, the three Fe-Mn oxyhydroxide fractions extracted by three different conventional methods, the second bulk sediment leachate, and the
Acknowledgements
This research was supported by the NRF Mid-Career Researcher Program (NRF-2011-0015174) and Basic Science Research Program (NRF-2014R1A1A3049836) funded by MSIP, Korea; the Gas Hydrate and Paleoceanographic Reconstruction in the Western Arctic Ocean (PE16062) and a KOPRI research grant (PE16010) funded by MOF, Korea; and the Integrated Ocean Drilling Program funded by the MLTMA, Korea. We thank M. Cook and K. Knudson for sharing oxygen isotope data, and H. Ashahi, K. Takahashi, B. Caissie for
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