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On the importance of opal, carbonate, and lithogenic clays in scavenging and fractionating 230Th, 231Pa and 10Be in the ocean
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Earth and Planetary Science Letters 220 (2004) 201^211 www.elsevier.com/locate/epsl
On the importance of opal, carbonate, and lithogenic clays in scavenging and fractionating 230Th, 231Pa and 10Be in the ocean Shangde Luo , Teh-Lung Ku Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA Received 9 May 2003; received in revised form 20 October 2003; accepted 24 December 2003
Abstract The influence of particle composition on the scavenging of 230 Th, 231 Pa, and 10 Be in the ocean is critically reexamined based on the particle-solution distribution coefficient (Kd ) data reported by Chase et al. [Earth Planet. Sci. Lett. 204 (2002) 215^229]. Our re-examination shows that the Kd s for these radionuclides are strongly influenced by the amount of lithogenic material (lithogenics) in particulates as reported previously by us, and that a strong correlation between the lithogenic and carbonate contents in open-ocean particulates results in an apparent, nonlinear correlation between %CaCO3 and Kd for each of the three radionuclides. Using a three-end-member model, we estimate that the respective Kd s (g/g) of 230 Th, 231 Pa, and 10 Be in the open ocean are: 2.3U108 , 1.2U107 , and 3.8U106 for lithogenics; 1.0U106 , 1.0U105 , and 3.0U104 for carbonate; and 2.5U105 , 1.2U106 , and 7.5U105 for opal, suggesting that lithogenics have a much stronger affinity for any of the three nuclides than does carbonate or opal. Lithogenics and carbonate preferentially scavenge 230 Th over 231 Pa and 10 Be, whereas opal slightly favors 231 Pa and 10 Be over 230 Th. Relative to lithogenics, carbonate plays a very limited role in fractionating 230 Th, 231 Pa, and 10 Be due to its orders-of-magnitude smaller Kd values for the three nuclides. Our results indicate that fractionations between Pa and Th and between Be and Th through particle scavenging are mainly determined by the opal-to-lithogenic ratio in particulates, rather than the opal-to-carbonate ratio suggested by Chase et al. : 2004 Elsevier B.V. All rights reserved. Keywords: Th-230; Pa-231; Be-10; scavenging; particle composition; distribution coe⁄cient
1. Introduction
ticles (lithogenics) have a far greater a⁄nity for Th and 231 Pa than biogenic particles of CaCO3 and opaline compositions. They also suggest that the observed fractionation between the two uranium-decay products in the sea is largely due to the preferential scavenging of 230 Th over 231 Pa by particulate lithogenics. Studies on cosmogenic 10 Be also indicate a strong a⁄nity of 10 Be to clay minerals [3^6] and that clay particles preferentially scavenge 230 Th over 10 Be [7]. These ¢nd230
Studying sediment-trap material and bottom sediments in the Paci¢c and Atlantic oceans, Luo and Ku [2] suggest that lithogenic clay par-
* Corresponding author. E-mail address: [email protected] (S. Luo).
0012-821X / 04 / $ ^ see front matter : 2004 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(04)00027-5
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ings greatly complicate, if not discourage, the use of 231 Pa/230 Th and 10 Be/230 Th ratios as proxies for paleoproductivity proposed earlier [8,9]. The importance of lithogenic particulates in scavenging Th, Pa and Be and in causing their fractionation has been challenged recently [1]. Using data on 230 Th, 231 Pa and 10 Be concentrations in sediment-trap-collected particles and in overlying water columns, Chase et al. [1] examine the empirical relationships between the particle-solution distribution coe⁄cient (Kd ) and particle composition in terms of percentages of lithogenics, carbonate, opal and organic carbon. Finding no correlation between %lithogenics and Kd s of the three radionuclides, they refute the concept that lithogenics are the primary agent responsible for scavenging 230 Th and 231 Pa from seawater. They conclude that : (1) CaCO3 has an a⁄nity for Th at least as strong as lithogenic phases do, and (2) the Th^Pa fractionation in the sea can be solely attributed to biogenic particulates of carbonate and opal, but not lithogenics. The resolution of the above apparently con£icting opinions on the role of lithogenic vs. biogenic particles in scavenging 230 Th, 231 Pa, and 10 Be in the ocean is important. It impacts not only the proposed use of 231 Pa/230 Th and 10 Be/230 Th as paleoceanographic/biogeochemical proxies, but also the understanding of the marine geochemistry of radionuclides and trace elements in general. In this regard, the wealth of information presented by Chase et al. [1] provides us with the opportunity to search for such a resolution by a careful re-examination of their data. In this paper, we propose a di¡erent method to examine the in£uence of particle composition on the oceanic scavenging of 230 Th, 231 Pa, and 10 Be based on the Kd data reported by Chase et al. [1]. We shall focus our examination on Chase et al. data from the central equatorial Paci¢c and Southern Ocean obtained during the US JGOFS equatorial Paci¢c (EqPac) and Southern Ocean (AESOPS) programs, as these are the main data base for their conclusions stated above. We will show that the aforementioned con£ict between Chase et al. [1] and Luo and Ku [2] can be resolved if a three-end-member system is taken into account. Employing several lines of reasoning,
our analysis below lends further credence to the suggestion [2] that lithogenics play a dominant role in fractionating and scavenging Th, Pa, and Be in the ocean.
2. Re-analysis of the Chase et al. [1] data 2.1. Strong correlation between Kd (Th) and lithogenic content Chase et al. [1] point out that the in£uence of particle composition on nuclide scavenging can be assessed using the ¢eld-based (in situ) distribution coe⁄cient (Kd ), de¢ned as the ratio of the nuclide concentration in particulates to that dissolved in seawater [10^12]. They estimate Kd for 230 Th, 231 Pa, and 10 Be by dividing the nuclide concentrations of sediment-trap-collected particles (atoms/g particulate) by those of sea waters (atoms/g seawater) in which the traps are deployed. Their estimates, together with data on major components of the particulates [13^15], have been reported in tabulated form in the Background Data Set of [1]. Plotting log Kd for Th, Pa, and Be vs. the lithogenic content of particulate matter and noting only weak or statistically insigni¢cant correlations, Chase et al. [1] question lithogenic phases as being primarily responsible for scavenging Th and Pa from seawater [2]. However, as we will show below, their questioning appears to arise from a faulty method of data analysis. In their plots (cf. ¢g. 1B of [1]), most of the open-ocean data points fall essentially on the y-axis, thus preventing any meaningful examination of the relationship between Kd (Th) and lithogenic content in the open ocean. In addition to the lack of a physical basis for their semi-log plots, which implicitly assume Kd to be an exponential function of particle composition, such plots are not called for by the data set inasmuch as both %lithogenics and Kd (Th) in the open ocean vary by similar orders of magnitude. In fact, the variation of %lithogenics is even greater than that of Kd (Th) if the data from marginal seas are included. We thus re-plot on linear^linear scales the central EqPac and AESOPS data in Fig. 1. The plot shows that a strong, linear and positive correlation (R2 = 0.97)
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between Kd (Th) and lithogenic content (fl ) indeed exists and that both the EqPac and AESOPS data assume a similar relationship. It should be noted that the observed Kd (Th) for high-%lithogenics particulates, mostly from the marginal seas (not shown, see [1]), is much smaller than suggested by our Fig. 1 relationship. This may re£ect that the speci¢c surface area (i.e., surface area-to-volume ratio) of a particulate exerts an important control on its Kd (Th). The coarser coastal-ocean lithogenics generally have much less speci¢c surface area, hence a smaller end-member Kd (Th), than do the ¢ner clay particles in the open ocean. Furthermore, Honeyman et al. [12] have shown that Kd (Th) decreases with increasing concentration of particulates in seawater and its value is ca. two orders of magnitude lower in the coastal seas than in the open ocean. By using a Browning-pumping model, Honeyman and Santschi [16] suggest that such a decrease of Kd (Th) is attributable to the occurrence of colloids in seawater. It is also noteworthy that the reported lithogenic contents of Chase et al. [1] are Ti-based at EqPac sites and Th-based at AESOPS sites, with the assumption that the lithogenics contain 0.4% of Ti or 10 ppm of Th. To correct for any possible systematic errors between the two reporting schemes, we compare the Ti-based estimates of Chase et al. [1] with those from Th measurements of Luo and Ku [2] on splits of the same sediment-
Fig. 1. Distribution coe⁄cient of Th, Kd (Th), as a function of lithogenic fraction (fl ) of particulate matter. The solid line represents a linear regression of the data.
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Fig. 2. Plot showing that the Th-based lithogenic contents are systematically higher than the Ti-based ones (denoted respectively by Th and Ti in parentheses). The relationship is used to convert the Ti-based estimates to Th-based ones (see text). Data are from those EqPac sediment-trap samples in which the lithogenic contents have been estimated by Chase et al. [1] (Ti-based) and Luo and Ku [2] (Th-based).
trap samples from EqPac (Fig. 2). The comparison shows that the Th-based lithogenic contents are higher than the Ti-based ones, with the di¡erence increasing with decreasing Ti contents. Since the 230 Th and 231 Pa measurements made on these samples by the two groups are in agreement, the di¡erence cannot be attributed to interlaboratory calibration in the Th measurements. Rather, it is likely that Th and Ti do not reside in exactly the same phase, e.g., Th may reside in ¢ne-grained clays and Ti in refractory phases [17]. As nuclide adsorption is mainly associated with ¢ne-grained particles, we choose to use Th as a proxy for lithogenics by converting the EqPac Ti-based estimates [1] into Th-based ones using the following correction factors: 4.3, 2.8, 1.8, 1.4 for fl (Tibased) in the ranges of 6 0.3%, 0.3^0.4%, 0.4^ 0.6%, and s 0.4%, respectively. The strong positive Kd (Th)^fl correlation shown in Fig. 1 bears two implications. First, it implies that lithogenics have a much stronger af¢nity for Th than do biogenic carbonate and opal. Fig. 1 shows that the end-member Kd (Th)s are (2.2 O 0.1)U108 g/g for fl = 1 and (2.7 O 0.7)U105 g/g for fl = 0, consistent with the results of Luo and Ku’s [2] two-component mixing model showing that the Kd (Th) in open-ocean lithogenics is about 500 times that of biogenic particles in the central equatorial Paci¢c. Second, the good line-
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arity in the Kd (Th)^fl relationship suggests constancy for the lithogenic end-member Kd (Th) that is hardly a¡ected by the presence of particulate carbonate. Since there is a strong, non-linear correlation between %lithogenics and %carbonate in the particulates (shown in Section 2.3), the good Kd (Th)^f1 linearity would not hold up if carbonate had an a⁄nity for Th at least as strong as that of lithogenic phases, as Chase et al. [1] have maintained. 2.2. Can carbonate be a strong scavenger of in the ocean?
230
Th
To assess the role of carbonate in 230 Th scavenging, two fundamental questions need be answered : (1) How can one constrain the end-member Kd (Th)s of carbonate and lithogenics if these two components are highly correlated ? (2) Why does Kd (Th) increase exponentially with %CaCO3 [1]? To answer question (1), we start with the following three-end-member mixing model to simultaneously constrain the Kd (Th)s in both carbonate and lithogenic phases: K d ¼ K d;l f l þ K d;c f c þ K d;o f o
ð1Þ
where Kd is the distribution coe⁄cient of a radionuclide, with subscript l, c, and o denoting lithogenics, carbonate, and opal, respectively, and f is the weight fraction of the corresponding end member in particulates. (We neglect the Kd (Th) of organic carbon as in [1].) Re-arranging: K d ¼
K d 3K d;o f o fl ¼ K d;c þ K d;l fc fc
ð2Þ
where Kd * is the opal-corrected, carbonate-normalized distribution coe⁄cient, in£uenced only by the lithogenic-to-carbonate ratio (fl /fc ) in particulates. Eq. 2 indicates that there should be no statistically signi¢cant correlation between Kd * and (fl /fc ) (i.e., the second term on the righthand side of Eq. 2 would be negligible) if the carbonate had a strong a⁄nity for Th. Kd *(Th) can be computed from the observed Kd (Th) and fo [1] by assuming that the biogenic end-member Kd (Th) derived from Fig. 1 approximates that of pure opal, i.e., Kd;o (Th)W(2.7 O 0.7)U105 g/g. Eq. 2 predicts a linear Kd *^(fl /fc ) relationship from
Fig. 3. Opal-corrected, carbonate-normalized distribution coe⁄cient of Th, Kd *(Th), as a function of lithogenic-to-carbonate ratio (fl /fc ) in particulate matter. The solid line represents a linear regression of the data.
which the relative importance of carbonate vs. lithogenics in Th scavenging can be assessed from the intercept (Kd;c ) and slope (Kd;l ). As shown in Fig. 3 for the data from the EqPac and AESOPS sites, there is a strong positive correlation between Kd * and (fl /fc ), with the good linearity indicating relatively constant end-member Kd (Th)s in the open ocean. From the Fig. 3 plots, we obtain Kd;c = (8.7 O 5.8)U105 g/g and Kd;l = (2.0 O 0.1)U108 g/g, indicating that the af¢nity of carbonate for Th is ca. two to three orders smaller than that of open-ocean lithogenics. The estimate of Kd;l is virtually identical to the (2.2 O 0.1)U108 g/g ¢gure derived from the Fig. 1 approach, pointing to the fact that lithogenics have so strong an a⁄nity for Th that the Kd (Th) vs. fl relationship of Fig. 1 is virtually una¡ected by the small di¡erence in Kd (Th)s between opal and carbonate. We conclude that carbonate cannot be a strong scavenger of 230 Th in the ocean. 2.3. Correlation between Kd (Th) and CaCO3 in particulates: causes and implications To address the second question posed above on the observed exponential increase of Kd (Th) with %CaCO3 [1], we plot log (%lithogenics) and log Kd (Th) as a function of %CaCO3 in Fig. 4a,b, respectively, for comparison. As shown, log (%lithogenics) and log Kd (Th) are both strongly and positively correlated with %CaCO3 . Furthermore, the two correlations exhibit comparable
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creases with increasing particle £ux, resulting in a positive and non-linear correlation between %lithogenics and %carbonate as seen in Fig. 4a. From the AESOPS and EqPac data on particle composition [1], we estimate fo +fc +fl = 0.83 O 0.07; the O 0.07 uncertainty is attributed mainly to the variability of organic content. Applying this relationship and that of Fig. 4a to Eq. 1, we have: K d ¼ 0:83K d;o þ f c ðK d;c 3K d;o Þþ 0:00213eð2:68f c Þ ðK d;l 3K d;o Þ
Fig. 4. (a) Distribution coe⁄cient of Th, Kd (Th), and (b) lithogenic content (%) as a function of carbonate content (%) in particulate matter. The solid lines represent the leastsquares ¢tting of the data by an exponential function. The two plots show a similar slope and correlation coe⁄cient.
slopes and correlation coe⁄cients. Together with the strong linear correlation between Kd (Th) and fl shown earlier (Fig. 1), they clearly suggest that it is the lithogenics, not carbonate, that give rise to high Kd (Th) values for open-ocean particulates. In other words, the observed non-linear relationship between Kd (Th) and %CaCO3 is the result of a strong correlation between carbonate and lithogenics, not a strong a⁄nity of Th for carbonate as suggested [1]. The observed positive correlation between %lithogenics and %CaCO3 in the pelagic ocean where lithogenics is only a minor component in particulates may be explained in terms of changes in surface productivity. CaCO3 and opal are two major components of particulate matter there [13,14]. They sum up to a relatively high but constant V80%, implying a negative correlation between the two, percentage-wise. High particle £ux (productivity) and opal content generally go hand-in-hand in the Southern Ocean [18,19] as well as at the EqPac sites [14] even though CaCO3 dominates the particle £ux in the latter area. An increase in productivity would thus lead to a decrease of both %carbonate and %lithogenics while increasing the £ux of both opal and carbonate. As a consequence, the lithogenics/carbonate ratio de-
ð3Þ
This equation depicts that Kd would be linearly correlated with fc only if Kd;l was small (e.g., Kd;l WKd;o ). This implies that the importance of lithogenics in scavenging Th is re£ected by the non-linearity between Kd (Th) and fc . Further, Eq. 3 provides a simultaneous constraint on the end-member Kd (Th)s for opal, carbonate, and lithogenics since these parameters are uniquely determined by the intercept, slope, and curvature
Fig. 5. Distribution coe⁄cients (Kd ) of (a) Th, (b) Pa, and (c) Be as a function of carbonate content (%) in particulate matter. The end-member Kd s of each nuclide in opal (Kd;o ), carbonate (Kd;c ), and lithogenics (Kd;l ) are determined by least-squares ¢ts of the data to Eq. 3 (solid lines).
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of the Kd (Th)^fc relationship. Fig. 5a shows the least-squares ¢t to Eq. 3 of the observed Kd (Th) and fc , from which we obtain Kd;l (Th) = 2.6U108 g/g, Kd;c (Th) = 1.2U106 g/g, and Kd;o (Th) = 2.2U 105 g/g. The uncertainties in these estimates come from two major sources: the Fig. 4a relationship between %carbonate and %lithogenics and the assumption of fo +fc +fl = 0.83. (Neglecting the Kd (Th) of organics may also contribute some uncertainty.) Nonetheless, the above estimates agree within O 10^15% with those derived from Figs. 1 and 3 respectively. Averaging results from the three methods gives (2.5 O 0.3)U105 g/g for Kd;o (Th), (1.0 O 0.2)U106 g/g for Kd;c (Th), and (2.3 O 0.3)U108 g/g for Kd;l (Th). While Kd (Th) of carbonate di¡ers from that of opal by only a factor of V4, it is at least two orders smaller than Kd (Th) of lithogenics. Therefore, compared to lithogenics, both carbonate and opal are much less e¡ective in scavenging Th in the sea.
A least-squares ¢t of the observed Kd (Pa) and fc to Eq. 3 gives Kd;l (Pa) = 1.2U107 g/g, Kd;c (Pa) = 1.0U105 g/g, and Kd;o (Pa) = 1.2U106 g/g (Fig. 5b). A similar ¢tting made for Kd (Be) and fc gives Kd;l (Be) = 3.8U106 g/g, Kd;c (Be) = 3.0U104 g/g, and Kd;o (Be) = 7.5U105 g/g (Fig. 5c). The uncertainties involved are comparable to those for the Kd (Th)s, judging from the similarity in their rootmean-square di¡erences shown in Fig. 5. These estimates are to be further con¢rmed later in Section 3.2 from a plot of Pa/Th or Be/Th as a function of particle composition. It appears that both Pa and Be have strong a⁄nities to lithogenics, with their Kd s in lithogenics about one to two orders of magnitude higher than in opal and carbonate.
3. Discussion 3.1. In£uence of particle composition on radionuclide scavenging
2.4. End-member Kd s for Pa and Be in marine particulates To assess the e¡ect of particle composition on the oceanic scavenging of 231 Pa and 10 Be, the ¢rst two methods (i.e., Figs. 1 and 3) may not apply. This is because, though lithogenics have strong a⁄nities for Pa and Be [2^6], these two elements have a much higher a⁄nity for opal than for carbonate such that their scavenging in high-opal, low-lithogenics regions such as the Southern Ocean is controlled by the opal content in particulates [20^22]. No simple linear relationships between Kd and fl or between Kd * and (fl /fc ) can be expected for 231 Pa and 10 Be. However, the third (i.e., Eq. 3) method is well suited for the assessment, especially since opal and carbonate have very di¡erent a⁄nities for 231 Pa or 10 Be.
Table 1 summarizes our estimates of the endmember Kd s of Th, Pa, and Be in opal, carbonate, and lithogenics. From these Kd values and under the conditions of fo +fc +fl W0.83 and f l ¼ 0:00213eð2:68f c Þ (Fig. 4a) for the data set, we derive in Fig. 6 the percentages of 230 Th, 231 Pa, and 10 Be in particulate phases of opal, carbonate and lithogenics as a function of major composition (e.g., %CaCO3 ). The results clearly show that s 70% of 230 Th is carried by lithogenics (Fig. 6a), even though lithogenics in open-ocean particulates often amount to a few percent or less. The results also suggest that in high-opal, low-carbonate areas, such as the Southern Ocean, opal is an important conveyor of 231 Pa and 10 Be. However, in high-carbonate, low-opal areas, such as the
Table 1 Estimates of the end-member Kd s for Th, Pa and Be, and the fractionation factors F(Pa/Th) and F(Be/Th) computed from the Kd s
Opal Carbonate Lithogenics
Kd (Th) (106 g/g)
Kd (Pa) (106 g/g)
Kd (Be) (106 g/g)
F(Pa/Th)
F(Be/Th)
V0.25 V1.0 V230
V1.2 V0.10 V12
V0.75 V0.03 V3.8
V4.8 V0.1 V0.05
V3.0 V0.03 V0.02
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Fig. 6. Percentages of (a) 230 Th, (b) 231 Pa, and (c) 10 Be in di¡erent particulate phases: lithogenics (solid lines), carbonate (dashed lines), and opal (dotted lines), as a function of carbonate content (%) in particulate matter. These plots were derived from the modeled parameters listed in Table 1 by de¢ning that %nuclide equals (Kd;o fo )/(Kd;o fo +Kd;c fc +Kd;l fl )U100 (in opal), (Kd;c fc )/(Kd;o fo +Kd;c fc +Kd;l fl )U100 (in carbonate), and (Kd;l f1 )/(Kd;o fo +Kd;c fc +Kd;l fl )U100 (in lithogenics), where fl is calculated from fc using the Fig. 4a relationship and fo = (0.833fc 3fl ). (Note that the CaCO3 content is rarely greater than 80%; see Fig. 4.)
equatorial Paci¢c where carbonate constitutes s 60% of the particulates with a carbonate/opal ratio of mostly V3 [14], lithogenics override opal to dominate the scavenging of both 231 Pa and 10 Be (Fig. 6b,c). Divergent views have been expressed as to determining which particulate phase is chie£y responsible for scavenging 231 Pa and 10 Be. Several studies suggest opal to be the phase [20^24], while others point to lithogenics [2^6]. A study of the 10 Be rain rate in the Southern Ocean leads to the conclusion that the rate is controlled by £uxes of lithogenics as well as biogenic opal [6]. In this study, we show that there is no con£ict among these observations. As lithogenics have a much stronger a⁄nity (higher Kd ) for both 231 Pa and 10 Be than does opal, it is possible that scavenging
207
of 231 Pa and 10 Be is controlled by opal at one site and by lithogenics at the other, depending on particulate opal-to-lithogenics ratios at a given site. However, this feature is not resolvable by using a two-end-member (carbonate vs. opal) model excluding lithogenics as [1] have attempted. It should be noted that the EqPac and AESOPS sites represent areas of high productivity and low lithogenic input, and hence are among the lowest in terms of radionuclide scavenging by lithogenics in the ocean. Even in areas south of the Polar Front in the Southern Ocean where the lithogenic rain rate is low, one can see a strong positive correlation between the lithogenic and 10 Be rain rates [6]. The scavenging by lithogenics is expected to be more important at low-opal, high-lithogenics ocean margins, e.g., in the western margin of the North Atlantic where enhanced scavenging is associated with the high lithogenic particle £ux from the continents ([7]; also see discussions below on Pa/Th and Be/Th ratios from the MidAtlantic Bight (MAB)). However, as aforementioned, the lithogenic detritus of ocean margins could well have their Kd s that are smaller than we obtained (Table 1) for the ¢ner-grained clays in open-ocean particulates due to the di¡erences in particulate surface area-to-volume ratio and particulate concentration in seawater, etc. Our estimates on end-member Kd s in particles (Table 1) may not necessarily agree with thermodynamically determined Kd s in the laboratory, nor provide insights as to the real cause for the large di¡erences in the estimated Kd s among different particulates. It is possible that the minor phases or impurities on the surface of particles, such as iron oxides and some organic compounds (e.g., polysaccharides, see Quigley et al. [25]) on clays may play an important role in adsorbing radionuclides. As pointed out earlier, the large Kd s in open-ocean lithogenics may also be related to the high speci¢c surface area of clays. Therefore, caution must be exercised when Kd values are used for assessing the particle’s capacity for radionuclide scavenging in an absolute sense. To avoid this potential problem, we discuss below the radionuclide scavenging/fractionation based on the ratio of Kd s between two di¡erent radionuclides.
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3.2. Radionuclide fractionation As in previous studies (e.g., [1,2]), we de¢ne fractionation factor, F, as the ratio of Kd s between two di¡erent radionuclides, e.g., F(Pa/ and F(Be/Th) = Kd (Be)/ Th) = Kd (Pa)/Kd (Th) Kd (Th) (note that Kd (Th) is used as a denominator). Plots of F(Pa/Th) (Fig. 7a) and F(Be/Th) (Fig. 7b) against fl /fb (lithogenic to biogenic ratio; fb = 13fl ) shows that both F(Pa/Th) and F(Be/Th) decrease signi¢cantly with increasing fl /fb . The strong non-linear, negative correlations indicate that the F(Pa/Th) and F(Be/Th) of pure lithogenics (fl /fb = r) must be at least one order smaller than that of biogenic particles (fl /fb = 0). Fig. 7 also shows that for fl /fb s 0.01, both F(Pa/Th) and F(Be/Th) are relatively constant (insensitive to fl /fb ) and can be approximated by their endmember values for pure lithogenics ^ additional evidence pointing to the strong a⁄nities of lithogenics for any of the three radionuclides 230 Th, 231 Pa, and 10 Be (i.e., their Kd s in lithogenics are
so large that the radionuclides occur primarily in lithogenic phases when fl /fb s 0.01). F(Pa/Th) at the lithogenics-dominated coastalocean sites at the MAB is much lower than in the open ocean (Fig. 7a), but it is in good agreement with that extrapolated from the open-ocean data points even though the lithogenic Kd at MAB may be orders of magnitude smaller than the open-ocean lithogenic Kd . The data o¡er no support for the suggestion [1] that low F(Pa/Th) is associated with low opal-to-carbonate ratio in particulates as the MAB particulates contain much higher opal-to-carbonate ratios than do the carbonate-rich, open-ocean particulates from the EqPac sites [1]. Rather, as can be seen from Fig. 7a inset, the low F(Pa/Th) is mainly attributable to the high lithogenic-to-biogenic ratio (fl /fb ). It shows that for a wide range of the fl /fb ratios from coastal to open oceans, the data basically follow a single relationship determined by the two common end-members : F(Pa/Th)W0.1 at fl /fb = r and F(Pa/Th) s 1 at fl /fb = 0. While the Fig. 7 plots enable the assessment of lithogenics vs. non-lithogenics in fractionating Th and Pa, they o¡er no information on the individual role of opal vs. carbonate in the fractionation [2]. Table 1 indicates that while opal preferentially scavenges Pa and Be over Th, both carbonate and lithogenics favor scavenging Th over Pa and Be. By using a model similar to that of Luo and Ku [2], we show below that the extrapolated F(Pa/Th) and F(Be/Th) values for fl /fb = 0 (Fig. 7) could well represent their values for pure opal (Table 1). Assuming that carbonate and lithogenics have the same F(Pa/Th) (Table 1), one can relate F(Pa/Th) to fl /fb ratio by the relationship : 1 F ¼ F l þ ðF o 3F l Þ 1 þ L ðf l =f b Þ
Fig. 7. Fractionation factors (F) for (a) Pa/Th and (b) Be/Th as a function of lithogenic-to-biogenic ratio (fl /fb ) in particulate matter, showing a systematic decrease of F(Pa/Th) and F(Be/Th) with increasing fl /fb . For fl /fb s 0.01, F(Pa/Th) and F(Be/Th) are insensitive to fl /fb and can be approximated by their values for the lithogenic end-member (fl /fb = r). Data in insets show that F(Pa/Th)s at the marginal sea (e.g., MAB) are consistent with those extrapolated from the openocean data. In contrast, F(Be/Th)s at MAB are signi¢cantly higher than the extrapolated values (see text for explanation).
ð4Þ
where F stands for F(Pa/Th) with subscripts l and o denoting lithogenics and opal, respectively, and parameter L = [Kd;l (Th)/Kd;o (Th)](1+K)/(fo /fb ); K = the amount of 230 Th in carbonate relative to that in lithogenics. Eq. 4 shows that Fo and Fl can be determined by extrapolating fl /fb to 0 and r, respectively. The curvature of the relationship (Fig. 7a) is governed by L, or by Kd;l (Th)/Kd;o (Th) and fo /fb as KI1 (Fig. 6a). If F(Pa/Th) is plotted
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against fl /fo (Fig. 8a), the curvature will not be a¡ected by fo /fb , as seen by rewriting Eq. 4 as: F ¼ F l þ ðF o 3F l Þ
1 1 þ L ðf l =f o Þ
ð5Þ
where L* = [Kd;l (Th)/Kd;o (Th)](1+K). Thus, assuming KW0.2 (Fig. 6a), one can constrain F(Pa/Th)o , F(Pa/Th)l and Kd;l (Th)/Kd;o (Th) by ¢tting the observed F(Pa/Th) and fl /fo to Eq. 5. As shown in Fig. 8a, Eq. 5 is well ¢tted not only by the openocean data, but also by data from the ocean-margin site of MAB, giving F(Pa/Th)o = 5.0, F(Pa/ Th)l = 0.075 and Kd;l (Th)/Kd;o (Th) = 950. A similar ¢tting can be made for the open-ocean F(Be/Th) and fl /fo data, giving F(Be/Th)o = 3.9, F(Be/ Th)l = 0.01 and Kd;l (Th)/Kd;o (Th) = 930 (Fig. 8b). The essentially identical Kd;l (Th)/Kd;o (Th) ratios derived from the two independent ¢ttings provide a validation for the model. The above estimates also reinforce the Table 1 results, lending further credence to the three-end-member, in situ Kd models discussed earlier. It should be pointed out that unlike F(Pa/Th), the F(Be/Th) values at MAB are signi¢cantly higher than the ‘expected’ values (V0.02) for the lithogenic end-member (Fig. 8b). There is evidence that signi¢cant amounts of lithogenics at
Fig. 8. Fractionation factors (F) of (a) Pa/Th and (b) Be/Th as a function of lithogenic-to-opal ratio (fl /fo ) in particulate matter. The solid lines represent a least-square ¢t of the open-ocean data to Eq. 5, giving: (a) F(Pa/Th)o = 5.0, F(Pa/ Th)l = 0.075, and Kd;l (Th)/Kd;o (Th) = 950 (assuming KW0.2) and (b) F(Be/Th)o = 3.9, F(Be/Th)l = 0.01 and Kd;l (Th)/ Kd;o (Th) = 930 (assuming KW0.2).
209
MAB originate from sediments remobilized from continental shelves [1,26]. As a result of the more intense boundary scavenging of 10 Be than of 230 Th and the high production ratio of 10 Be/ 230 Th on continental shelves, the remobilized sediments could have 10 Be/230 Th ratios signi¢cantly higher than those in deeper water where sediment traps were deployed [26]. If so, the high 10 Be/ 230 Th from the remobilized sediments would be the likely cause for the observed high F(Be/Th) at MAB. Such sediments would have a negligible e¡ect on F(Pa/Th) because : (1) the production ratio of 231 Pa/230 Th is independent of water depth, and (2) boundary scavenging of 231 Pa at MAB is insigni¢cant [26,27].
4. Summary The oceanic scavenging behavior of trace elements in general, and 230 Th, 231 Pa, and 10 Be in particular, is a subject of considerable interest. Because of relatively well-known input functions for the three radioisotopes, they have been used individually or collectively as tracers for a variety of oceanographic processes (e.g., [8,9,27^29]). To validate such tracer utility, Luo and Ku [2] examined the in£uence of particle composition on radionuclide scavenging by emphasizing the important role of lithogenous clay particles in the removal and fractionation of 230 Th and 231 Pa. Their emphasis is reinforced by the present reanalysis of the relatively large data sets from the equatorial Paci¢c and Southern Ocean reported recently by Chase et al. [1]. Our analysis o¡ers no support for the hypothesis [1] that the fractionation between Th and Pa can be solely explained by relative amounts of carbonate and opal in marine particulates without the involvement of scavenging by lithogenics. The Chase et al. [1] data set indicates that the Kd of 230 Th, 231 Pa, and 10 Be each is at least two orders smaller for carbonate than for lithogenic material. The amounts of these radionuclides scavenged by carbonate are too small (relative to those scavenged by lithogenics) to signi¢cantly in£uence the F(Pa/Th) and F(Be/Th) in particulates. In analyzing the same data set from the EqPac
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and AESOPS sites, the key di¡erence between this study and Chase et al. [1] is that we have recognized the strong linear correlation between Kd (Th) and lithogenic content in particulates and the strong non-linear correlation between the particulate carbonate and lithogenic content. These correlations provide evidence that radionuclide scavenging in the open ocean is strongly a¡ected by the amount of particulate lithogenics. They also explain why the bulk Kd s of 230 Th, 231 Pa and 10 Be are non-linearly correlated with the CaCO3 content in particulates, and make it possible to quantify the role of carbonate vs. lithogenics in the scavenging of these radionuclides. Neglecting the lithogenics and treating marine particulates at the EqPac and AESOPS sites as a two-end-member (opal vs. carbonate) system [1] may result in misleading conclusions. For example, a high Kd;c (Th) value of V1U107 [1] thus derived for calcite endmember was remarked as a surprise [30]. In view of the present analysis, it seems that no surprise is in store as such an elevated value for marine calcite may well be non-existent.
Acknowledgements We thank Peter Santschi and Martin Frank for their thoughtful comments, which signi¢cantly helped improve the presentation. This research was supported by the US National Science Foundation under Grant OCE-0117895.[BOYLE]
References [1] Z. Chase, R.F. Anderson, M.Q. Fleisher, P.W. Kubik, The in£uence of particle composition and particle £ux on scavenging of Th, Pa, and Be in the ocean, Earth Planet. Sci. Lett. 204 (2002) 215^229. [2] S. Luo, T.L. Ku, Oceanic 231 Pa/230 Th ratio in£uenced by particle composition and remineralization, Earth Planet. Sci. Lett. 167 (1999) 183^195. [3] J.R. Southon, T.L. Ku, D.E. Nelson, J.L. Reyss, J.C. Duplessy, J.S. Vogel, 10 Be in a deep-sea core: implications regarding 10 Be production changes over the past 420 ka, Earth Planet. Sci. Lett. 85 (1987) 356^364. [4] P. Sharma, R. Mahannah, W.S. Moore, T.L. Ku, J.R. Southon, Transport of 10 Be and 9 Be in the ocean, Earth Planet. Sci. Lett. 86 (1987) 69^76.
[5] W.U. Henken-Mellies, J. Beer, F. Heller, K.J. Hsu, C. Shen, G. Bonani, H.J. Hofmann, M. Suter, W. Wol£i, 10 Be and 9 Be in south Atlantic DSDP Site 519: relation to geomagnetic reversals and sediment composition, Earth Planet. Sci. Lett. 98 (1990) 267^276. [6] M. Frank, R. Gersonde, M. Rutgers van der Loe¡, G. Bohrmann, C.C. Nurnberg, P.W. Kubik, M. Suter, A. Mangini, Similar glacial and interglacial export bioproductivity in the Atlantic sector of the Southern Ocean: multiproxy evidence and implications for glacial atmospheric CO2 , Paleoceanography 15 (2000) 642^658. [7] S. Luo, K.L. Ku, L. Wang, J.R. Southon, S.P. Lund, M. Schwartz, 26 Al, 10 Be, and U-Th isotopes in Blake Outer Ridge sediments: implications for past changes in boundary scavenging, Earth Planet. Sci. Lett. 185 (2001) 135^ 147. [8] N. Kumar, R. Gwiazda, R.F. Anderson, P.N. Froelich, 231 Pa/230 Th ratios in sediments as a proxy for past changes in Southern Ocean productivity, Nature 362 (1993) 45^48. [9] N. Kumar, R.F. Anderson, R.A. Mortlock, P.N. Froelich, P.W. Kubik, B. Dittrich-Hannen, M. Suter, Increased productivity and export production in the glacial Southern Ocean, Nature 378 (1996) 675^680. [10] M.P. Bacon, R.F. Anderson, Distribution of thorium isotopes between dissolved and particulate forms in the deep sea, J. Geophys. Res. 87 (1982) 2045^2056. [11] Y. Nozaki, H.S. Yang, M. Yamada, Scavenging of thorium in the ocean, J. Geophys. Res. 92 (1987) 772^778. [12] B.D. Honeyman, L.S. Balistrieri, J.W. Murray, Oceanic trace-metal scavenging ^ the importance of particle composition, Deep-Sea Res. 87 (1988) 227^246. [13] S. Honjo, R. Franscois, S. Manganini, J. Dymond, R. Collier, Particle £uxes to the interior of the Southern Ocean in the western Paci¢c sector along 170‡W, DeepSea Res. II 47 (2000) 3521^3548. [14] S. Honjo, J. Dymond, R. Collier, S. Manganini, Export production of particles to the interior of the equatorial Paci¢c during 1992 EqPac Experiment, Deep-Sea Res. II 42 (1995) 831^870. [15] P.E. Biscaye, R.F. Anderson, B.L. Deck, Fluxes of particles and constituents to the Eastern United States continental slope and rise: SEEP-I, Cont. Shelf Res. 8 (1988) 855^904. [16] B.D. Honeyman, P.H. Santschi, A browning-pumping model for oceanic trace metal scavenging: eveidence from Th isotopes, J. Mar. Res. 47 (1989) 951^992. [17] R.W. Murray, M. Leinen, Scavenging excess aluminum and its relationship to bulk titanium in biogenic sediment from the central equatorial Paci¢c Ocean, Geochim. Cosmochim. Acta 60 (1996) 3869^3878. [18] K.O. Buesseler, The decoupling of production and particulate export in the southern ocean, Glob. Biogeochem. Cycles 12 (1998) 297^310. [19] D.M. Nelson, P. Treguer, M.A. Brzezinski, A. Leynaert, B. Queguiner, Production and dissolution of biogenic silica in the southern ocean: Revised global estimates, com-
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[20] [21]
[22]
[23]
[24]
[25]
parison with regional data and relationship to biogenic sedimentation, Glob. Biogeochem. Cycles 9 (1995) 359^ 372. D.J. DeMaster, The Marine Budget of Silica and 32 Si, Ph.D. Thesis, Yale University, New Haven, CT, 1979. Y. Lao, R.F. Anderson, W.S. Broecker, S.E. Trumbore, H.J. Hofmann, W. Wo«i£i, Transport and burial rates of 10 Be and 231 Pa in the Paci¢c Ocean during the Holocene period, Earth Planet. Sci. Lett. 113 (1992) 173^189. H.J. Walter, M.M. Rutgers van der Loe¡, H. Hoeltzen, Enhanced scavenging of 231 Pa relative to 230 Th in the South Atlantic south of the Polar Front: implications for the use of the 231 Pa/230 Th ratio as a paleoproductivity proxy, Earth Planet. Sci. Lett. 149 (1997) 85^100. K. Taguchi, K. Harada, S. Tsunogai, Particulate removal of 230 Th and 231 Pa in the biologically productive northern North Paci¢c, Earth Planet. Sci. Lett. 93 (1989) 223^232. Y. Lao, R.F. Anderson, W.S. Broecker, H.J. Hofmann, W. Wo«l£i, Particulate £uxes of 230 Th, 231 Pa, and 10 Be in the northeastern Paci¢c Ocean, Geochim. Cosmochim. Acta 57 (1993) 205^217. M.S. Quigley, P.H. Santschi, C.-C. Hung, L. Guo, B.D. Honeyman, Importance of polysaccharides for 234 Th complexation to marine organic matter, Limnol. Oceanogr. 47 (2002) 367^377.
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[26] R.F. Anderson, M.Q. Fleisher, P.E. Biscaye, N. Kumar, B. Ditrich, P. Kubik, M. Suter, Anomalous boundary scavenging in the Middle Atlantic Bight: evidence from 230 Th, 231 Pa, 10 Be and 210 Pb, Deep-Sea Res. II 41 (1994) 537^561. [27] E.F. Yu, R. Francois, M.P. Bacon, Similar rates of modern and last-glacial ocean thermohaline circulation inferred from radiochemical data, Nature 379 (1996) 689^ 694. [28] R. Francois, M.P. Bacon, D. Suman, Thorium-230 pro¢ling in deep-sea sediments: High-resolution records of £ux and dissolution of carbonate in the equatorial Atlantic during the last 24,000 years, Paleoceanography 5 (1990) 761^787. [29] M. Frank, J.-D. Eckhardt, A. Eisenhauer, P.W. Kubik, B. Dittrich-Hannen, A. Mangini, Beryllium 10, thorium 230 and protactinium 231 in Galapagos Microplate sediments: implications for hydrothermal activity and paleoproductivity changes during the last 100,000 years, Paleoceanography 9 (1994) 559^578. [30] G.M. Henderson, R.F. Anderson, The U-series toolbox for paleoceanography, in: B. Bourdon, G.M. Henderson, C.C. Lundstrom, S.P. Turner (Eds.), Uranium-Series Geochemistry, Rev. Mineral. Geochem. 52 (2003) 493^ 531.
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On the importance of opal, carbonate, and lithogenic clays in scavenging and fractionating 230Th, 231Pa and 10Be in the ocean Shangde Luo , Teh-Lung Ku Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA Received 9 May 2003; received in revised form 20 October 2003; accepted 24 December 2003
Abstract The influence of particle composition on the scavenging of 230 Th, 231 Pa, and 10 Be in the ocean is critically reexamined based on the particle-solution distribution coefficient (Kd ) data reported by Chase et al. [Earth Planet. Sci. Lett. 204 (2002) 215^229]. Our re-examination shows that the Kd s for these radionuclides are strongly influenced by the amount of lithogenic material (lithogenics) in particulates as reported previously by us, and that a strong correlation between the lithogenic and carbonate contents in open-ocean particulates results in an apparent, nonlinear correlation between %CaCO3 and Kd for each of the three radionuclides. Using a three-end-member model, we estimate that the respective Kd s (g/g) of 230 Th, 231 Pa, and 10 Be in the open ocean are: 2.3U108 , 1.2U107 , and 3.8U106 for lithogenics; 1.0U106 , 1.0U105 , and 3.0U104 for carbonate; and 2.5U105 , 1.2U106 , and 7.5U105 for opal, suggesting that lithogenics have a much stronger affinity for any of the three nuclides than does carbonate or opal. Lithogenics and carbonate preferentially scavenge 230 Th over 231 Pa and 10 Be, whereas opal slightly favors 231 Pa and 10 Be over 230 Th. Relative to lithogenics, carbonate plays a very limited role in fractionating 230 Th, 231 Pa, and 10 Be due to its orders-of-magnitude smaller Kd values for the three nuclides. Our results indicate that fractionations between Pa and Th and between Be and Th through particle scavenging are mainly determined by the opal-to-lithogenic ratio in particulates, rather than the opal-to-carbonate ratio suggested by Chase et al. : 2004 Elsevier B.V. All rights reserved. Keywords: Th-230; Pa-231; Be-10; scavenging; particle composition; distribution coe⁄cient
1. Introduction
ticles (lithogenics) have a far greater a⁄nity for Th and 231 Pa than biogenic particles of CaCO3 and opaline compositions. They also suggest that the observed fractionation between the two uranium-decay products in the sea is largely due to the preferential scavenging of 230 Th over 231 Pa by particulate lithogenics. Studies on cosmogenic 10 Be also indicate a strong a⁄nity of 10 Be to clay minerals [3^6] and that clay particles preferentially scavenge 230 Th over 10 Be [7]. These ¢nd230
Studying sediment-trap material and bottom sediments in the Paci¢c and Atlantic oceans, Luo and Ku [2] suggest that lithogenic clay par-
* Corresponding author. E-mail address: [email protected] (S. Luo).
0012-821X / 04 / $ ^ see front matter : 2004 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(04)00027-5
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ings greatly complicate, if not discourage, the use of 231 Pa/230 Th and 10 Be/230 Th ratios as proxies for paleoproductivity proposed earlier [8,9]. The importance of lithogenic particulates in scavenging Th, Pa and Be and in causing their fractionation has been challenged recently [1]. Using data on 230 Th, 231 Pa and 10 Be concentrations in sediment-trap-collected particles and in overlying water columns, Chase et al. [1] examine the empirical relationships between the particle-solution distribution coe⁄cient (Kd ) and particle composition in terms of percentages of lithogenics, carbonate, opal and organic carbon. Finding no correlation between %lithogenics and Kd s of the three radionuclides, they refute the concept that lithogenics are the primary agent responsible for scavenging 230 Th and 231 Pa from seawater. They conclude that : (1) CaCO3 has an a⁄nity for Th at least as strong as lithogenic phases do, and (2) the Th^Pa fractionation in the sea can be solely attributed to biogenic particulates of carbonate and opal, but not lithogenics. The resolution of the above apparently con£icting opinions on the role of lithogenic vs. biogenic particles in scavenging 230 Th, 231 Pa, and 10 Be in the ocean is important. It impacts not only the proposed use of 231 Pa/230 Th and 10 Be/230 Th as paleoceanographic/biogeochemical proxies, but also the understanding of the marine geochemistry of radionuclides and trace elements in general. In this regard, the wealth of information presented by Chase et al. [1] provides us with the opportunity to search for such a resolution by a careful re-examination of their data. In this paper, we propose a di¡erent method to examine the in£uence of particle composition on the oceanic scavenging of 230 Th, 231 Pa, and 10 Be based on the Kd data reported by Chase et al. [1]. We shall focus our examination on Chase et al. data from the central equatorial Paci¢c and Southern Ocean obtained during the US JGOFS equatorial Paci¢c (EqPac) and Southern Ocean (AESOPS) programs, as these are the main data base for their conclusions stated above. We will show that the aforementioned con£ict between Chase et al. [1] and Luo and Ku [2] can be resolved if a three-end-member system is taken into account. Employing several lines of reasoning,
our analysis below lends further credence to the suggestion [2] that lithogenics play a dominant role in fractionating and scavenging Th, Pa, and Be in the ocean.
2. Re-analysis of the Chase et al. [1] data 2.1. Strong correlation between Kd (Th) and lithogenic content Chase et al. [1] point out that the in£uence of particle composition on nuclide scavenging can be assessed using the ¢eld-based (in situ) distribution coe⁄cient (Kd ), de¢ned as the ratio of the nuclide concentration in particulates to that dissolved in seawater [10^12]. They estimate Kd for 230 Th, 231 Pa, and 10 Be by dividing the nuclide concentrations of sediment-trap-collected particles (atoms/g particulate) by those of sea waters (atoms/g seawater) in which the traps are deployed. Their estimates, together with data on major components of the particulates [13^15], have been reported in tabulated form in the Background Data Set of [1]. Plotting log Kd for Th, Pa, and Be vs. the lithogenic content of particulate matter and noting only weak or statistically insigni¢cant correlations, Chase et al. [1] question lithogenic phases as being primarily responsible for scavenging Th and Pa from seawater [2]. However, as we will show below, their questioning appears to arise from a faulty method of data analysis. In their plots (cf. ¢g. 1B of [1]), most of the open-ocean data points fall essentially on the y-axis, thus preventing any meaningful examination of the relationship between Kd (Th) and lithogenic content in the open ocean. In addition to the lack of a physical basis for their semi-log plots, which implicitly assume Kd to be an exponential function of particle composition, such plots are not called for by the data set inasmuch as both %lithogenics and Kd (Th) in the open ocean vary by similar orders of magnitude. In fact, the variation of %lithogenics is even greater than that of Kd (Th) if the data from marginal seas are included. We thus re-plot on linear^linear scales the central EqPac and AESOPS data in Fig. 1. The plot shows that a strong, linear and positive correlation (R2 = 0.97)
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between Kd (Th) and lithogenic content (fl ) indeed exists and that both the EqPac and AESOPS data assume a similar relationship. It should be noted that the observed Kd (Th) for high-%lithogenics particulates, mostly from the marginal seas (not shown, see [1]), is much smaller than suggested by our Fig. 1 relationship. This may re£ect that the speci¢c surface area (i.e., surface area-to-volume ratio) of a particulate exerts an important control on its Kd (Th). The coarser coastal-ocean lithogenics generally have much less speci¢c surface area, hence a smaller end-member Kd (Th), than do the ¢ner clay particles in the open ocean. Furthermore, Honeyman et al. [12] have shown that Kd (Th) decreases with increasing concentration of particulates in seawater and its value is ca. two orders of magnitude lower in the coastal seas than in the open ocean. By using a Browning-pumping model, Honeyman and Santschi [16] suggest that such a decrease of Kd (Th) is attributable to the occurrence of colloids in seawater. It is also noteworthy that the reported lithogenic contents of Chase et al. [1] are Ti-based at EqPac sites and Th-based at AESOPS sites, with the assumption that the lithogenics contain 0.4% of Ti or 10 ppm of Th. To correct for any possible systematic errors between the two reporting schemes, we compare the Ti-based estimates of Chase et al. [1] with those from Th measurements of Luo and Ku [2] on splits of the same sediment-
Fig. 1. Distribution coe⁄cient of Th, Kd (Th), as a function of lithogenic fraction (fl ) of particulate matter. The solid line represents a linear regression of the data.
203
Fig. 2. Plot showing that the Th-based lithogenic contents are systematically higher than the Ti-based ones (denoted respectively by Th and Ti in parentheses). The relationship is used to convert the Ti-based estimates to Th-based ones (see text). Data are from those EqPac sediment-trap samples in which the lithogenic contents have been estimated by Chase et al. [1] (Ti-based) and Luo and Ku [2] (Th-based).
trap samples from EqPac (Fig. 2). The comparison shows that the Th-based lithogenic contents are higher than the Ti-based ones, with the di¡erence increasing with decreasing Ti contents. Since the 230 Th and 231 Pa measurements made on these samples by the two groups are in agreement, the di¡erence cannot be attributed to interlaboratory calibration in the Th measurements. Rather, it is likely that Th and Ti do not reside in exactly the same phase, e.g., Th may reside in ¢ne-grained clays and Ti in refractory phases [17]. As nuclide adsorption is mainly associated with ¢ne-grained particles, we choose to use Th as a proxy for lithogenics by converting the EqPac Ti-based estimates [1] into Th-based ones using the following correction factors: 4.3, 2.8, 1.8, 1.4 for fl (Tibased) in the ranges of 6 0.3%, 0.3^0.4%, 0.4^ 0.6%, and s 0.4%, respectively. The strong positive Kd (Th)^fl correlation shown in Fig. 1 bears two implications. First, it implies that lithogenics have a much stronger af¢nity for Th than do biogenic carbonate and opal. Fig. 1 shows that the end-member Kd (Th)s are (2.2 O 0.1)U108 g/g for fl = 1 and (2.7 O 0.7)U105 g/g for fl = 0, consistent with the results of Luo and Ku’s [2] two-component mixing model showing that the Kd (Th) in open-ocean lithogenics is about 500 times that of biogenic particles in the central equatorial Paci¢c. Second, the good line-
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arity in the Kd (Th)^fl relationship suggests constancy for the lithogenic end-member Kd (Th) that is hardly a¡ected by the presence of particulate carbonate. Since there is a strong, non-linear correlation between %lithogenics and %carbonate in the particulates (shown in Section 2.3), the good Kd (Th)^f1 linearity would not hold up if carbonate had an a⁄nity for Th at least as strong as that of lithogenic phases, as Chase et al. [1] have maintained. 2.2. Can carbonate be a strong scavenger of in the ocean?
230
Th
To assess the role of carbonate in 230 Th scavenging, two fundamental questions need be answered : (1) How can one constrain the end-member Kd (Th)s of carbonate and lithogenics if these two components are highly correlated ? (2) Why does Kd (Th) increase exponentially with %CaCO3 [1]? To answer question (1), we start with the following three-end-member mixing model to simultaneously constrain the Kd (Th)s in both carbonate and lithogenic phases: K d ¼ K d;l f l þ K d;c f c þ K d;o f o
ð1Þ
where Kd is the distribution coe⁄cient of a radionuclide, with subscript l, c, and o denoting lithogenics, carbonate, and opal, respectively, and f is the weight fraction of the corresponding end member in particulates. (We neglect the Kd (Th) of organic carbon as in [1].) Re-arranging: K d ¼
K d 3K d;o f o fl ¼ K d;c þ K d;l fc fc
ð2Þ
where Kd * is the opal-corrected, carbonate-normalized distribution coe⁄cient, in£uenced only by the lithogenic-to-carbonate ratio (fl /fc ) in particulates. Eq. 2 indicates that there should be no statistically signi¢cant correlation between Kd * and (fl /fc ) (i.e., the second term on the righthand side of Eq. 2 would be negligible) if the carbonate had a strong a⁄nity for Th. Kd *(Th) can be computed from the observed Kd (Th) and fo [1] by assuming that the biogenic end-member Kd (Th) derived from Fig. 1 approximates that of pure opal, i.e., Kd;o (Th)W(2.7 O 0.7)U105 g/g. Eq. 2 predicts a linear Kd *^(fl /fc ) relationship from
Fig. 3. Opal-corrected, carbonate-normalized distribution coe⁄cient of Th, Kd *(Th), as a function of lithogenic-to-carbonate ratio (fl /fc ) in particulate matter. The solid line represents a linear regression of the data.
which the relative importance of carbonate vs. lithogenics in Th scavenging can be assessed from the intercept (Kd;c ) and slope (Kd;l ). As shown in Fig. 3 for the data from the EqPac and AESOPS sites, there is a strong positive correlation between Kd * and (fl /fc ), with the good linearity indicating relatively constant end-member Kd (Th)s in the open ocean. From the Fig. 3 plots, we obtain Kd;c = (8.7 O 5.8)U105 g/g and Kd;l = (2.0 O 0.1)U108 g/g, indicating that the af¢nity of carbonate for Th is ca. two to three orders smaller than that of open-ocean lithogenics. The estimate of Kd;l is virtually identical to the (2.2 O 0.1)U108 g/g ¢gure derived from the Fig. 1 approach, pointing to the fact that lithogenics have so strong an a⁄nity for Th that the Kd (Th) vs. fl relationship of Fig. 1 is virtually una¡ected by the small di¡erence in Kd (Th)s between opal and carbonate. We conclude that carbonate cannot be a strong scavenger of 230 Th in the ocean. 2.3. Correlation between Kd (Th) and CaCO3 in particulates: causes and implications To address the second question posed above on the observed exponential increase of Kd (Th) with %CaCO3 [1], we plot log (%lithogenics) and log Kd (Th) as a function of %CaCO3 in Fig. 4a,b, respectively, for comparison. As shown, log (%lithogenics) and log Kd (Th) are both strongly and positively correlated with %CaCO3 . Furthermore, the two correlations exhibit comparable
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creases with increasing particle £ux, resulting in a positive and non-linear correlation between %lithogenics and %carbonate as seen in Fig. 4a. From the AESOPS and EqPac data on particle composition [1], we estimate fo +fc +fl = 0.83 O 0.07; the O 0.07 uncertainty is attributed mainly to the variability of organic content. Applying this relationship and that of Fig. 4a to Eq. 1, we have: K d ¼ 0:83K d;o þ f c ðK d;c 3K d;o Þþ 0:00213eð2:68f c Þ ðK d;l 3K d;o Þ
Fig. 4. (a) Distribution coe⁄cient of Th, Kd (Th), and (b) lithogenic content (%) as a function of carbonate content (%) in particulate matter. The solid lines represent the leastsquares ¢tting of the data by an exponential function. The two plots show a similar slope and correlation coe⁄cient.
slopes and correlation coe⁄cients. Together with the strong linear correlation between Kd (Th) and fl shown earlier (Fig. 1), they clearly suggest that it is the lithogenics, not carbonate, that give rise to high Kd (Th) values for open-ocean particulates. In other words, the observed non-linear relationship between Kd (Th) and %CaCO3 is the result of a strong correlation between carbonate and lithogenics, not a strong a⁄nity of Th for carbonate as suggested [1]. The observed positive correlation between %lithogenics and %CaCO3 in the pelagic ocean where lithogenics is only a minor component in particulates may be explained in terms of changes in surface productivity. CaCO3 and opal are two major components of particulate matter there [13,14]. They sum up to a relatively high but constant V80%, implying a negative correlation between the two, percentage-wise. High particle £ux (productivity) and opal content generally go hand-in-hand in the Southern Ocean [18,19] as well as at the EqPac sites [14] even though CaCO3 dominates the particle £ux in the latter area. An increase in productivity would thus lead to a decrease of both %carbonate and %lithogenics while increasing the £ux of both opal and carbonate. As a consequence, the lithogenics/carbonate ratio de-
ð3Þ
This equation depicts that Kd would be linearly correlated with fc only if Kd;l was small (e.g., Kd;l WKd;o ). This implies that the importance of lithogenics in scavenging Th is re£ected by the non-linearity between Kd (Th) and fc . Further, Eq. 3 provides a simultaneous constraint on the end-member Kd (Th)s for opal, carbonate, and lithogenics since these parameters are uniquely determined by the intercept, slope, and curvature
Fig. 5. Distribution coe⁄cients (Kd ) of (a) Th, (b) Pa, and (c) Be as a function of carbonate content (%) in particulate matter. The end-member Kd s of each nuclide in opal (Kd;o ), carbonate (Kd;c ), and lithogenics (Kd;l ) are determined by least-squares ¢ts of the data to Eq. 3 (solid lines).
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of the Kd (Th)^fc relationship. Fig. 5a shows the least-squares ¢t to Eq. 3 of the observed Kd (Th) and fc , from which we obtain Kd;l (Th) = 2.6U108 g/g, Kd;c (Th) = 1.2U106 g/g, and Kd;o (Th) = 2.2U 105 g/g. The uncertainties in these estimates come from two major sources: the Fig. 4a relationship between %carbonate and %lithogenics and the assumption of fo +fc +fl = 0.83. (Neglecting the Kd (Th) of organics may also contribute some uncertainty.) Nonetheless, the above estimates agree within O 10^15% with those derived from Figs. 1 and 3 respectively. Averaging results from the three methods gives (2.5 O 0.3)U105 g/g for Kd;o (Th), (1.0 O 0.2)U106 g/g for Kd;c (Th), and (2.3 O 0.3)U108 g/g for Kd;l (Th). While Kd (Th) of carbonate di¡ers from that of opal by only a factor of V4, it is at least two orders smaller than Kd (Th) of lithogenics. Therefore, compared to lithogenics, both carbonate and opal are much less e¡ective in scavenging Th in the sea.
A least-squares ¢t of the observed Kd (Pa) and fc to Eq. 3 gives Kd;l (Pa) = 1.2U107 g/g, Kd;c (Pa) = 1.0U105 g/g, and Kd;o (Pa) = 1.2U106 g/g (Fig. 5b). A similar ¢tting made for Kd (Be) and fc gives Kd;l (Be) = 3.8U106 g/g, Kd;c (Be) = 3.0U104 g/g, and Kd;o (Be) = 7.5U105 g/g (Fig. 5c). The uncertainties involved are comparable to those for the Kd (Th)s, judging from the similarity in their rootmean-square di¡erences shown in Fig. 5. These estimates are to be further con¢rmed later in Section 3.2 from a plot of Pa/Th or Be/Th as a function of particle composition. It appears that both Pa and Be have strong a⁄nities to lithogenics, with their Kd s in lithogenics about one to two orders of magnitude higher than in opal and carbonate.
3. Discussion 3.1. In£uence of particle composition on radionuclide scavenging
2.4. End-member Kd s for Pa and Be in marine particulates To assess the e¡ect of particle composition on the oceanic scavenging of 231 Pa and 10 Be, the ¢rst two methods (i.e., Figs. 1 and 3) may not apply. This is because, though lithogenics have strong a⁄nities for Pa and Be [2^6], these two elements have a much higher a⁄nity for opal than for carbonate such that their scavenging in high-opal, low-lithogenics regions such as the Southern Ocean is controlled by the opal content in particulates [20^22]. No simple linear relationships between Kd and fl or between Kd * and (fl /fc ) can be expected for 231 Pa and 10 Be. However, the third (i.e., Eq. 3) method is well suited for the assessment, especially since opal and carbonate have very di¡erent a⁄nities for 231 Pa or 10 Be.
Table 1 summarizes our estimates of the endmember Kd s of Th, Pa, and Be in opal, carbonate, and lithogenics. From these Kd values and under the conditions of fo +fc +fl W0.83 and f l ¼ 0:00213eð2:68f c Þ (Fig. 4a) for the data set, we derive in Fig. 6 the percentages of 230 Th, 231 Pa, and 10 Be in particulate phases of opal, carbonate and lithogenics as a function of major composition (e.g., %CaCO3 ). The results clearly show that s 70% of 230 Th is carried by lithogenics (Fig. 6a), even though lithogenics in open-ocean particulates often amount to a few percent or less. The results also suggest that in high-opal, low-carbonate areas, such as the Southern Ocean, opal is an important conveyor of 231 Pa and 10 Be. However, in high-carbonate, low-opal areas, such as the
Table 1 Estimates of the end-member Kd s for Th, Pa and Be, and the fractionation factors F(Pa/Th) and F(Be/Th) computed from the Kd s
Opal Carbonate Lithogenics
Kd (Th) (106 g/g)
Kd (Pa) (106 g/g)
Kd (Be) (106 g/g)
F(Pa/Th)
F(Be/Th)
V0.25 V1.0 V230
V1.2 V0.10 V12
V0.75 V0.03 V3.8
V4.8 V0.1 V0.05
V3.0 V0.03 V0.02
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Fig. 6. Percentages of (a) 230 Th, (b) 231 Pa, and (c) 10 Be in di¡erent particulate phases: lithogenics (solid lines), carbonate (dashed lines), and opal (dotted lines), as a function of carbonate content (%) in particulate matter. These plots were derived from the modeled parameters listed in Table 1 by de¢ning that %nuclide equals (Kd;o fo )/(Kd;o fo +Kd;c fc +Kd;l fl )U100 (in opal), (Kd;c fc )/(Kd;o fo +Kd;c fc +Kd;l fl )U100 (in carbonate), and (Kd;l f1 )/(Kd;o fo +Kd;c fc +Kd;l fl )U100 (in lithogenics), where fl is calculated from fc using the Fig. 4a relationship and fo = (0.833fc 3fl ). (Note that the CaCO3 content is rarely greater than 80%; see Fig. 4.)
equatorial Paci¢c where carbonate constitutes s 60% of the particulates with a carbonate/opal ratio of mostly V3 [14], lithogenics override opal to dominate the scavenging of both 231 Pa and 10 Be (Fig. 6b,c). Divergent views have been expressed as to determining which particulate phase is chie£y responsible for scavenging 231 Pa and 10 Be. Several studies suggest opal to be the phase [20^24], while others point to lithogenics [2^6]. A study of the 10 Be rain rate in the Southern Ocean leads to the conclusion that the rate is controlled by £uxes of lithogenics as well as biogenic opal [6]. In this study, we show that there is no con£ict among these observations. As lithogenics have a much stronger a⁄nity (higher Kd ) for both 231 Pa and 10 Be than does opal, it is possible that scavenging
207
of 231 Pa and 10 Be is controlled by opal at one site and by lithogenics at the other, depending on particulate opal-to-lithogenics ratios at a given site. However, this feature is not resolvable by using a two-end-member (carbonate vs. opal) model excluding lithogenics as [1] have attempted. It should be noted that the EqPac and AESOPS sites represent areas of high productivity and low lithogenic input, and hence are among the lowest in terms of radionuclide scavenging by lithogenics in the ocean. Even in areas south of the Polar Front in the Southern Ocean where the lithogenic rain rate is low, one can see a strong positive correlation between the lithogenic and 10 Be rain rates [6]. The scavenging by lithogenics is expected to be more important at low-opal, high-lithogenics ocean margins, e.g., in the western margin of the North Atlantic where enhanced scavenging is associated with the high lithogenic particle £ux from the continents ([7]; also see discussions below on Pa/Th and Be/Th ratios from the MidAtlantic Bight (MAB)). However, as aforementioned, the lithogenic detritus of ocean margins could well have their Kd s that are smaller than we obtained (Table 1) for the ¢ner-grained clays in open-ocean particulates due to the di¡erences in particulate surface area-to-volume ratio and particulate concentration in seawater, etc. Our estimates on end-member Kd s in particles (Table 1) may not necessarily agree with thermodynamically determined Kd s in the laboratory, nor provide insights as to the real cause for the large di¡erences in the estimated Kd s among different particulates. It is possible that the minor phases or impurities on the surface of particles, such as iron oxides and some organic compounds (e.g., polysaccharides, see Quigley et al. [25]) on clays may play an important role in adsorbing radionuclides. As pointed out earlier, the large Kd s in open-ocean lithogenics may also be related to the high speci¢c surface area of clays. Therefore, caution must be exercised when Kd values are used for assessing the particle’s capacity for radionuclide scavenging in an absolute sense. To avoid this potential problem, we discuss below the radionuclide scavenging/fractionation based on the ratio of Kd s between two di¡erent radionuclides.
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3.2. Radionuclide fractionation As in previous studies (e.g., [1,2]), we de¢ne fractionation factor, F, as the ratio of Kd s between two di¡erent radionuclides, e.g., F(Pa/ and F(Be/Th) = Kd (Be)/ Th) = Kd (Pa)/Kd (Th) Kd (Th) (note that Kd (Th) is used as a denominator). Plots of F(Pa/Th) (Fig. 7a) and F(Be/Th) (Fig. 7b) against fl /fb (lithogenic to biogenic ratio; fb = 13fl ) shows that both F(Pa/Th) and F(Be/Th) decrease signi¢cantly with increasing fl /fb . The strong non-linear, negative correlations indicate that the F(Pa/Th) and F(Be/Th) of pure lithogenics (fl /fb = r) must be at least one order smaller than that of biogenic particles (fl /fb = 0). Fig. 7 also shows that for fl /fb s 0.01, both F(Pa/Th) and F(Be/Th) are relatively constant (insensitive to fl /fb ) and can be approximated by their endmember values for pure lithogenics ^ additional evidence pointing to the strong a⁄nities of lithogenics for any of the three radionuclides 230 Th, 231 Pa, and 10 Be (i.e., their Kd s in lithogenics are
so large that the radionuclides occur primarily in lithogenic phases when fl /fb s 0.01). F(Pa/Th) at the lithogenics-dominated coastalocean sites at the MAB is much lower than in the open ocean (Fig. 7a), but it is in good agreement with that extrapolated from the open-ocean data points even though the lithogenic Kd at MAB may be orders of magnitude smaller than the open-ocean lithogenic Kd . The data o¡er no support for the suggestion [1] that low F(Pa/Th) is associated with low opal-to-carbonate ratio in particulates as the MAB particulates contain much higher opal-to-carbonate ratios than do the carbonate-rich, open-ocean particulates from the EqPac sites [1]. Rather, as can be seen from Fig. 7a inset, the low F(Pa/Th) is mainly attributable to the high lithogenic-to-biogenic ratio (fl /fb ). It shows that for a wide range of the fl /fb ratios from coastal to open oceans, the data basically follow a single relationship determined by the two common end-members : F(Pa/Th)W0.1 at fl /fb = r and F(Pa/Th) s 1 at fl /fb = 0. While the Fig. 7 plots enable the assessment of lithogenics vs. non-lithogenics in fractionating Th and Pa, they o¡er no information on the individual role of opal vs. carbonate in the fractionation [2]. Table 1 indicates that while opal preferentially scavenges Pa and Be over Th, both carbonate and lithogenics favor scavenging Th over Pa and Be. By using a model similar to that of Luo and Ku [2], we show below that the extrapolated F(Pa/Th) and F(Be/Th) values for fl /fb = 0 (Fig. 7) could well represent their values for pure opal (Table 1). Assuming that carbonate and lithogenics have the same F(Pa/Th) (Table 1), one can relate F(Pa/Th) to fl /fb ratio by the relationship : 1 F ¼ F l þ ðF o 3F l Þ 1 þ L ðf l =f b Þ
Fig. 7. Fractionation factors (F) for (a) Pa/Th and (b) Be/Th as a function of lithogenic-to-biogenic ratio (fl /fb ) in particulate matter, showing a systematic decrease of F(Pa/Th) and F(Be/Th) with increasing fl /fb . For fl /fb s 0.01, F(Pa/Th) and F(Be/Th) are insensitive to fl /fb and can be approximated by their values for the lithogenic end-member (fl /fb = r). Data in insets show that F(Pa/Th)s at the marginal sea (e.g., MAB) are consistent with those extrapolated from the openocean data. In contrast, F(Be/Th)s at MAB are signi¢cantly higher than the extrapolated values (see text for explanation).
ð4Þ
where F stands for F(Pa/Th) with subscripts l and o denoting lithogenics and opal, respectively, and parameter L = [Kd;l (Th)/Kd;o (Th)](1+K)/(fo /fb ); K = the amount of 230 Th in carbonate relative to that in lithogenics. Eq. 4 shows that Fo and Fl can be determined by extrapolating fl /fb to 0 and r, respectively. The curvature of the relationship (Fig. 7a) is governed by L, or by Kd;l (Th)/Kd;o (Th) and fo /fb as KI1 (Fig. 6a). If F(Pa/Th) is plotted
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against fl /fo (Fig. 8a), the curvature will not be a¡ected by fo /fb , as seen by rewriting Eq. 4 as: F ¼ F l þ ðF o 3F l Þ
1 1 þ L ðf l =f o Þ
ð5Þ
where L* = [Kd;l (Th)/Kd;o (Th)](1+K). Thus, assuming KW0.2 (Fig. 6a), one can constrain F(Pa/Th)o , F(Pa/Th)l and Kd;l (Th)/Kd;o (Th) by ¢tting the observed F(Pa/Th) and fl /fo to Eq. 5. As shown in Fig. 8a, Eq. 5 is well ¢tted not only by the openocean data, but also by data from the ocean-margin site of MAB, giving F(Pa/Th)o = 5.0, F(Pa/ Th)l = 0.075 and Kd;l (Th)/Kd;o (Th) = 950. A similar ¢tting can be made for the open-ocean F(Be/Th) and fl /fo data, giving F(Be/Th)o = 3.9, F(Be/ Th)l = 0.01 and Kd;l (Th)/Kd;o (Th) = 930 (Fig. 8b). The essentially identical Kd;l (Th)/Kd;o (Th) ratios derived from the two independent ¢ttings provide a validation for the model. The above estimates also reinforce the Table 1 results, lending further credence to the three-end-member, in situ Kd models discussed earlier. It should be pointed out that unlike F(Pa/Th), the F(Be/Th) values at MAB are signi¢cantly higher than the ‘expected’ values (V0.02) for the lithogenic end-member (Fig. 8b). There is evidence that signi¢cant amounts of lithogenics at
Fig. 8. Fractionation factors (F) of (a) Pa/Th and (b) Be/Th as a function of lithogenic-to-opal ratio (fl /fo ) in particulate matter. The solid lines represent a least-square ¢t of the open-ocean data to Eq. 5, giving: (a) F(Pa/Th)o = 5.0, F(Pa/ Th)l = 0.075, and Kd;l (Th)/Kd;o (Th) = 950 (assuming KW0.2) and (b) F(Be/Th)o = 3.9, F(Be/Th)l = 0.01 and Kd;l (Th)/ Kd;o (Th) = 930 (assuming KW0.2).
209
MAB originate from sediments remobilized from continental shelves [1,26]. As a result of the more intense boundary scavenging of 10 Be than of 230 Th and the high production ratio of 10 Be/ 230 Th on continental shelves, the remobilized sediments could have 10 Be/230 Th ratios signi¢cantly higher than those in deeper water where sediment traps were deployed [26]. If so, the high 10 Be/ 230 Th from the remobilized sediments would be the likely cause for the observed high F(Be/Th) at MAB. Such sediments would have a negligible e¡ect on F(Pa/Th) because : (1) the production ratio of 231 Pa/230 Th is independent of water depth, and (2) boundary scavenging of 231 Pa at MAB is insigni¢cant [26,27].
4. Summary The oceanic scavenging behavior of trace elements in general, and 230 Th, 231 Pa, and 10 Be in particular, is a subject of considerable interest. Because of relatively well-known input functions for the three radioisotopes, they have been used individually or collectively as tracers for a variety of oceanographic processes (e.g., [8,9,27^29]). To validate such tracer utility, Luo and Ku [2] examined the in£uence of particle composition on radionuclide scavenging by emphasizing the important role of lithogenous clay particles in the removal and fractionation of 230 Th and 231 Pa. Their emphasis is reinforced by the present reanalysis of the relatively large data sets from the equatorial Paci¢c and Southern Ocean reported recently by Chase et al. [1]. Our analysis o¡ers no support for the hypothesis [1] that the fractionation between Th and Pa can be solely explained by relative amounts of carbonate and opal in marine particulates without the involvement of scavenging by lithogenics. The Chase et al. [1] data set indicates that the Kd of 230 Th, 231 Pa, and 10 Be each is at least two orders smaller for carbonate than for lithogenic material. The amounts of these radionuclides scavenged by carbonate are too small (relative to those scavenged by lithogenics) to signi¢cantly in£uence the F(Pa/Th) and F(Be/Th) in particulates. In analyzing the same data set from the EqPac
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and AESOPS sites, the key di¡erence between this study and Chase et al. [1] is that we have recognized the strong linear correlation between Kd (Th) and lithogenic content in particulates and the strong non-linear correlation between the particulate carbonate and lithogenic content. These correlations provide evidence that radionuclide scavenging in the open ocean is strongly a¡ected by the amount of particulate lithogenics. They also explain why the bulk Kd s of 230 Th, 231 Pa and 10 Be are non-linearly correlated with the CaCO3 content in particulates, and make it possible to quantify the role of carbonate vs. lithogenics in the scavenging of these radionuclides. Neglecting the lithogenics and treating marine particulates at the EqPac and AESOPS sites as a two-end-member (opal vs. carbonate) system [1] may result in misleading conclusions. For example, a high Kd;c (Th) value of V1U107 [1] thus derived for calcite endmember was remarked as a surprise [30]. In view of the present analysis, it seems that no surprise is in store as such an elevated value for marine calcite may well be non-existent.
Acknowledgements We thank Peter Santschi and Martin Frank for their thoughtful comments, which signi¢cantly helped improve the presentation. This research was supported by the US National Science Foundation under Grant OCE-0117895.[BOYLE]
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