4.5 Nd in the oceans

The abundance of Nd in seawater is about a million times lower than in rocks, at ca. 3 parts per trillion (Goldberg et al., 1963; Piepgras et al., 1979). This can be attributed to effective scavenging of rare earths from seawater by particulate matter. In contrast, ions such as sodium have similar abundances in rocks and seawater. This led Goldberg et al. to propose that Nd has a very short residence time in seawater, possibly less than 300 yr, and less than the turnover rate of water in the oceans. As a result, we can expect Nd isotope systematics in seawater to be quite different from those of Sr (section 3.6.2), which has an ocean residence time of more than 2 Myr.

4.5.1 Modern seawater Nd

The very low Nd concentrations in seawater present significant analytical difficulties. In contrast, manganese nodules, which are believed to precipitate directly from seawater, have Nd contents up to hundreds of ppm. Consequently the early studies of O’Nions et al. (1978) and Piepgras et al. (1979) focussed principally on this material. Significant Nd isotopic variations were found between Mn nodules from different ocean basins (Fig. 4.39b) and attributed by Piepgras et al. to real variations in the isotopic composition of seawater.

Piepgras et al. justified their interpretation on the grounds that Mn nodules from a wide geographical area within each ocean mass had distinct but reproducible Nd isotope compositions. This was confirmed by the direct analysis of filtered ocean water samples (Fig. 4.39a), which were shown to be consistent with the isotopic composition of sea floor nodules from the same ocean basin. Direct Nd isotope analyses of four water samples from the Pacific (totalling 10 to 20 litres each) were presented by Piepgras et al. (1979), while analyses of Atlantic ocean water were presented by Piepgras and Wasserburg (1980).

Fig. 4.39. Histograms showing the ranges of , Nd displayed by: a) seawater; and b) manganese nodules from different ocean basins; relative to: c) major crustal reservoirs. After Piepgras and Wasserburg (1980).

Comparison of seawater isotope compositions with possible source reservoirs (Fig. 4.39c) suggested that Nd in Atlantic seawater is primarily continental in origin. This is consistent with a greater discharge of river water into the Atlantic. However, about 50% of Nd in Pacific seawater appears to be derived from basaltic crustal sources, either from erosion of mafic rocks or from some form of exchange with ocean floor basalts. Second-order isotopic variations were also seen within Atlantic Ocean water. Samples from depths of at least 1 km in the Sargasso Sea have very consistent , Nd compositions averaging !13.5, whereas shallower samples from depths of 300 and 50 m have more radiogenic , Nd values of !10.9 and !9.6 respectively. This implies isotopic stratification of Atlantic water masses, a conclusion consistent with long-established oceanographic observations of the Atlantic (Wust, 1924).

A more detailed Nd isotope study of the North Atlantic Ocean was performed by Piepgras and Wasserburg (1987) using water samples from five vertical sections. Contoured , Nd values (Fig. 4.40 a) are consistent with water masses recognised on the basis of salinity and temperature (Fig. 4.40 b). Surface water at mid-latitudes (SW) has , Nd values consistent with the dissolved Nd budgets of major rivers such as the Amazon and Mississippi (Piepgras and Wasserburg, 1987; Goldstein and Jacobsen, 1987). Outflow of water from the Mediterranean also has a similar composition (Piepgras and Wasserburg, 1983). In contrast, the major water body of the ocean, North Atlantic Deep Water (NADW), has very uniform unradiogenic Nd (, = !13.5). It is well known that this water largely originates from Arctic Intermediate Water (AIW), which has been shown by Stordal and Wasserburg (1986) to have , Nd as low as !25 in Baffin Bay. Therefore the , Nd composition of NADW must result from mixing of AIW and mid-latitude surface water. Finally, as NADW flows southward towards the equator, it becomes sandwiched between two tongues of water with intermediate , Nd, Antarctic Intermediate and Bottom water (AAIW, AABW, Fig. 4.40 b).

Fig. 4.40. Schematic longitudinal sections through the Atlantic Ocean to show: a) contoured Nd isotope variations in the North Atlantic; b) oceanographically established water masses for the whole Atlantic (with sample locations in part ‘a’ shown by dashed lines). After Piepgras and Wasserburg (1987).

Although the continental origin of Atlantic seawater Nd is well established, the origin of the radiogenic Nd in Pacific seawater is more problematical. The most obvious source is hydrothermal alteration of ocean floor basalts. However, the low REE contents of hydrothermal vent fluids (Michard et al., 1983) rule out a simple hydrothermal origin, as proposed for Sr and Pb (sections 3.6.2 and 5.6.2). It is possible that a diffuse alteration flux from the whole ocean floor contributes some radiogenic Nd. However, a more important source may be volcanic dust from circum-Pacific volcanoes (Albarede and Goldstein, 1992).

Wind-blown sediment has also been proposed as a major source for the continental Nd signatures in Atlantic Ocean water. This is supported by the regionality of the seawater Nd signatures, which can be tied in with prevailing wind directions. For example, unradiogenic Nd in the eastern North Atlantic can be traced to wind-blown particulates from the Sahara desert (Chester et al., 1979). Similar effects occur in the eastern North Pacific, which receives wind-blown particulates from the deserts of the American southeast (Albarede and Goldstein, 1992).

Dissolution of Nd from wind-blown dust can also solve a ‘Nd paradox’ arising from conflicting oceanic residence times of Nd based on elemental and isotopic data. This paradox arises from the fact that Nd concentrations of deep ocean water increase from Atlantic to Pacific, along a deep ocean current termed the ‘ocean conveyor belt’ (section 14.1.7). This implies a long residence time of Nd in seawater (possibly as high as 10 kyr). However, the isotopic variation between Atlantic and Pacific seawater Nd implies a short residence time, which must be less than the 1500 yr circulation time of the ‘conveyor belt’. To resolve this paradox, Bertam and Elderfiled (1993) proposed Nd exchange between the suspended and dissolved Nd budgets in seawater.

This exchange process was revealed in more detail by measuring oceanic depth profiles of particulate and dissolved Nd off western Africa, within the range of Saharan wind-borne particles (Tachikawa et al., 1999). This work revealed a large flux of Nd to surface ocean water, derived from dissolution of ‘lithogenic’ (i.e. clastic) particulates, largely of atmospheric origin. This input flux is balanced by an equally large output flux, comprising Nd which is adsorbed onto sinking biogenic and authigenic particulates produced in the water mass.

4.5.2 Ancient seawater Nd

Following the successful characterisation of the Nd isotope budget of the modern oceans, Shaw and Wasserburg (1985) evaluated various types of material as indicators of the Nd isotope composition of paleo-oceans. They found that carbonate and phosphate in living organisms were very low in Nd (with concentrations in the part per billion range), but that fossil carbonates and phosphates had concentrations in the tens to hundreds of ppb and ppm respectively. Shaw and Wasserburg attributed the elevated Nd contents of fossil carbonates largely to diagenetic remobilisation of detrital Nd, but they attributed the high Nd contents of ancient phosphates (conodonts, fish debris, lingulid brachiopods and inorganic phosphorites) to scavenging directly from seawater (after death). Several studies were made on this kind of material during the late 1980s, allowing a general understanding of the evolution of seawater Nd through time.

Keto and Jacobsen (1988) collated conodont and phosphorite data with analyses of fish teeth (Staudigel et al., 1985), ferromanganese coatings on forams (Palmer and Elderfield, 1986) and conodonts and lingulids (Keto and Jacobsen, 1987; 1988) to construct a paleo-seawater Nd curve for the Phanerozoic. Because the Pacific Ocean dominates the world ocean system (as did its predecessor, the Panthalassan Ocean), this was used to determine a global average evolution curve (Fig. 4.41a). This curve was then extended into the Precambrian (Fig. 4.41b) by the analysis of Archean and Proterozoic banded iron formations (BIF), which were argued to sample the Nd isotope composition of the Precambrian oceans (Jacobsen and Pimentel-Klose, 1988). Despite the lack of reliable paleogeographical information for this period of Earth history, it may be justifiable to assume world-wide homogenisation of Nd in seawater on the grounds that a smaller continental mass during the Precambrian presented less impediment to circulatory mixing of the oceans.

Fig. 4.41. Proposed global seawater evolution curve. a) For the Phanerozoic, based on phosphate samples from the Pacific)Panthalassa Ocean. b) For the Precambrian, based on banded iron formations. After Keto and Jacobsen (1988); Jacobsen and Pimentel-Klose (1988).

The Precambrian curve based on BIF data suggests that unlike the Phanerozoic, when continental run-off is the dominant influence, Archean seawater Nd was controlled by the weathering of mafic mantle-derived rocks. This is consistent the presumed smaller continental mass at that time. The Proterozoic is then a period of transition from the mantle-dominated regime of the Archean to the crust-dominated regime of the Phanerozoic.

4.5.3 Tertiary seawater Nd

Notwithstanding the significance of these advances in understanding the ancient oceans, the most powerful applications of Nd isotope analysis to oceanography have resulted from more detailed studies of seawater evolution during the Tertiary epoch, parallelling the detailed study of seawater Sr for this period (section 3.6.1). These studies have been revolutionised by the ability to measure continuous secular variations of Nd isotope composition from ferromanganese crusts.

Ferromanganese crusts grow on any exposed surface in the deep ocean at a rate of about 1 - 3 mm/Myr. Because of this very slow rate of growth they are easily swamped by sedimentation; but on elevated areas, such as seamounts and volcanic plateaux, ferromanganese crusts can grow unimpeded for more than 20 Myr (Ling et al., 1997). Furthermore, crusts growing on these features can sample the isotopic composition at different water depths, from as shallow as 850 m to abyssal depths of 5000 m (Reynolds et al., 1999). In order to use ferromangenese crusts as an inventory of past seawater Nd signatures it is necessary to measure the growth rates of crusts accurately. Consequently, this has been the focus of considerable research.

The most precise measurements of the growth rates of ferromangenese crusts are obtained from U-series isotopes (section 12.3.2). Because internal checks can be made using different U-series methods, these are also the most accurate data. However, these methods cannot reach beyond 400 kyr, whereas many crusts have grown for more than 20 Myr. An alternative approach attempted in early work on crusts was to use Sr isotope stratigraphy (section 3.6.1). This method was investigated by Ingram et al. (1990) and VonderHaar et al. (1995), and appeared to give reasonable growth rates on one or two Atlantic samples (e.g. Burton et al., 1997). However, more detailed studies (e.g. Ling et al., 1997; O’Nions et al., 1998) revealed inconsistencies with other dating techniques, so the method has now largely been abandoned.

An alternative dating technique with a range of 10 Myr involves the cosmogenic isotope ¹⁰Be (section 14.3.4). This method has proved quite reliable, especially when ¹⁰Be abundances are normalised with respect to ⁹Be (e.g. Ling et al., 1997). Beyond 10 Myr, the only method that has been proved reliable is cobalt dating. This is based on the assumption that ferromangenese crusts receive a constant input of cobalt with time, so that lower cobalt concentrations imply a faster growth rate, and vice versa. Hence, Frank et al. (1999a) showed that growth rates of three long-lived crusts, based on cobalt abundances, were consistent with growth rates extrapolated from the ¹⁰Be/⁹Be chronometer.

One of the most interesting observations from these studies, based initially on the analysis of one crust from the North Atlantic (Burton et al., 1997) and one from the Central Pacific (Ling et al., 1997), was the existence of a sharp change in Nd isotope composition in both ocean masses around 4 Myr ago (Fig. 4.42). This period had previously been identified from oxygen isotope evidence as one of increased salinity in the Caribbean, due to closure of the ‘Panama Gateway’ that had once linked the Pacific Ocean to the Caribbean. Hence, Burton et al. (1997) attributed inflections in the Nd isotope profiles around 4 Myr ago to changes in the global ocean circulation pattern (section 14.1.7), prompted by the closure of the Gateway.

Fig. 4.42. Preliminary Nd isotope data for Fe–Mn crusts from the Pacific and Atlantic oceans, showing possible inflection points in seawater evolution, relative to the estimated time of closure of the Panama gateway. After Burton et al. (1997)

Unfortunately, the Atlantic data in Fig. 4.42 were dated by the seawater Sr method, which was subsequently shown to give ages about double the true value. However, close examination of the data for the Atlantic ocean shows that there are actually two inflections in the curve, and that the 4 Myr timeline passes half way between them. Therefore, after recalibration of the profile to cosmogenic Be ages, the inflections moved to ca. 4 and 8 Myr ago, respectively, consistent with more recent measurements by Burton et al. (1999) and Frank et al. (1999b).

The new data (Fig. 4.43) continue to support the idea that there was a change in Atlantic Oceanic Nd signatures about 4 Myr ago, possibly due to the closure of the Panama Gateway. However, Reynolds et al. suggested that this closure may have been progressive, reflecting a gradual shallowing of the Gateway starting 8 Myr ago, which was finally completed 4 Myr ago. In addition, some other Atlantic ferromanganese crusts analysed by Reynolds et al. (1999) showed inflections at different times. Therefore, closure of the Gateway was probably not the only factor which led to changes in ocean circulation patterns over the past 10 Myr (Frank, 2002). Pb isotope ratios were also analysed in the same samples, but since Pb was significantly decoupled from Nd, the data will be discussed separately (section 5.6.2).

Fig. 4.43. Comparison of several Fe–Mn profiles for the Pacific and Atlantic oceans, relative to the time of closure of the Panama gateway. After Frank et al. (1999b)

4.5.4 Quaternary seawater Nd

The slow growth rates of ferromanganese crusts preclude their use to study short-term changes in seawater Nd signatures, such as might be found during Quaternary glacial cycles. Therefore, other types of material, capable of reliably recording short-term variations, were sought. Forams present an attractive prospect because they are widely distributed, their rapid rates of sedimentation can yield high-resolution profiles, and they are already linked to glacial cycles by stable isotope measurements. A problem is that forams become coated after accumulation on the seafloor with ferromanganese deposits whose Nd concentrations are much higher than those in the forams themselves. However, Vance and Burton (1999) showed that after removal of these coatings by leaching with a strong reducing agent, the original Nd isotope compositions of forams could successfully be recovered.

Burton and Vance (2000) applied this method to the analysis of forams in a 3 m long core from the northern Indian Ocean, covering the past 150 kyr. Several tests were done to check that the measured Nd isotope ratios were original rather than secondary. These included analysis of Mn/Ca ratios as a monitor of the effectiveness of the cleaning procedure, comparisons between two different fossil foram species, and comparisons of Nd content and isotope ratio with modern Indian Ocean forams. All these tests gave confidence that the method was recovering original seawater Nd signatures. When the resulting down-hole record of Nd isotope composition was examined (Fig. 4.44), an almost perfect mirror image of the oxygen isotope record was seen for the last glacial cycle. This suggests strongly that the Nd isotope record reflects climatic processes associated with the glacial cycle. Burton and Vance attributed the isotopic fluctuations to a balance between the supply of radiogenic Nd from the main body of the Indian Ocean and the supply of unradiogenic Nd from Himalayan erosion. Hence, if the monsoon was attenuated during the last glacial maximum, the result would be the more radiogenic Indian Ocean Nd signatures seen in Fig. 4.44

Fig. 4.44. Plot of a) Nd isotope ratio, and b) oxygen isotope ratio (per mil relative to PDB), in planktonic forams from a drill core in the northern Indian Ocean. After Burton and Vance (2000).

A somewhat surprising observation made by Burton and Vance (2000) was that uncleaned forams with ferromanganese coatings had the same isotopic signatures as cleaned forams, and that even bulk sediment samples gave a profile that was parallel to that for the cleaned samples, but offset 1 - 2 , units above it. This suggests that even dispersed ferromanganese oxides in sediment cores may be a viable record of past seawater Nd signatures. Based on this assumption, Rutberg et al. (2000) extracted Nd from bulk core sediments by leaching with a strong reducing agent. By this means they examined an 80 kyr record of seawater Nd in a sediment core from the southeast Atlantic Ocean, in the Cape Basin off South Africa. This is a critical location for understanding the behaviour of the ocean system because many tracer studies have shown that North Atlantic Deep Water (NADW) is mixed with Antarctic Bottom Water in the southern ocean to form Circum Polar Water, which is then exported to the Pacific. This forms the so-called ‘ocean conveyor belt’ (section 14.1.7).

Based on radiocarbon evidence (section 14.1.7) it is expected that the ocean conveyor belt was ‘turned off’ or reduced during the last glacial maximum (ca. 20 kyr ago). However, this model was challenged by evidence from U-series isotopes (section 12.3.6), which implied that the conveyor continued unabated during the glacial maximum. Nevertheless, U-series tracers are susceptible to disturbance by changes in biological production, whereas the Nd isotope system is less susceptible to this kind of disturbance. Therefore, Nd isotope data may help to resolve this conflict.

The study of Rutberg et al. (2000) provides preliminary data to address this problem, provided that the observed isotopic variations are original and not diagenetic. Evidence in support of their validity as original seawater compositions came from the preservation of typical seawater Sr isotope signatures in the analysed leachates, despite the presence of radiogenic Sr in coexisting detrital phases. Given this assumption, variations in Nd isotope ratio can be attributed to variations in the supply of NADW to the southern ocean. The fact that these variations are in step with climatically-controlled carbon isotope variations (Fig. 4.45) provides evidence to support changes in the strength of the conveyer belt between glacial and interglacial periods.

Fig. 4.45. Plot of Nd isotope data for leached ferromanganese phases from a drill core in the Southern Ocean ( ! ), compared with carbon isotope variations in benthic forams from the same core (solid line). After Rutberg et al. (2000).

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