the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jun 2026
Lead isotopes in deep sea sediments reveal alteration of the isotopic composition signal in the water column at highly productive sites in the South Atlantic Ocean
Abstract. Lead (Pb) isotope fingerprints in marine sediments are frequently used as tracers of natural or anthropogenic Pb sources. This assumes that Pb isotope ratios are not altered within the water column or sediments. Recent studies suggest reverse scavenging of Pb isotopes by biogenic particles during sinking through the water column, but it remains unclear if and how this affects the Pb isotope signal in deep sea sediments. This study examines Pb isotope records (206Pb/207Pb and 208Pb/207Pb) in pelagic sediments of a highly productive area in the South Atlantic Ocean and their relation to primary productivity as reflected by the silicon (Si) and total organic carbon (TOC) contents. Three cores were extracted at three different stations northwest and southeast of South Georgia and at different water depths (St9: 3796 m, St15: 8066 m, and St16: 5002 m). Downcore Pb and total organic carbon concentrations ranged from 4 to 12 µg g-1 and from 0.17 to 0.89 % respectively, with highest Pb concentrations in the core taken at the deepest water depth. Lead isotope signatures reflected Patagonian dust as the major Pb source. Lead isotope ratios in the sediment cores were generally larger than those of dissolved Pb in the water phase observed in previous studies, indicating a separation between Pb isotope pools in the water column. Distinct variations in Pb isotope ratios correlated with the content of Si and TOC, indicating an influence of primary productivity on Pb scavenging in the water column. Correlations of Pb concentrations and isotope ratios with TOC content were most pronounced at the deepest sampling station, but were less distinct at other stations presumably due to dilution by biogenic Si. Besides implications on the biogeochemical Pb marine cycle, our findings question the use of Pb isotope signatures in deep sea sediments for source tracing. Future studies combining water phase, particulate matter and sediment Pb isotope analyses should address how signatures of Pb isotope ratios are altered in the water phase during sinking and investigate fractionation processes during Pb burial in deep sea sediments.
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Status: open (until 25 Aug 2026)
- RC1: 'Comment on egusphere-2026-3608', Anonymous Referee #1, 21 Jul 2026 reply
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RC2: 'Comment on egusphere-2026-3608', Anonymous Referee #2, 29 Jul 2026
reply
The paper presents Pb isotope data for 3 marine sediment cores, which are interpreted with support from elemental concentration data. The data exhibit some interesting features, namely variations in Pb isotope composition that co-vary with Si and total organic carbon (TOC) concentrations and inversely with elements diagnostic of lithogenic inputs. The authors interpret this to reflect the alteration of the Pb isotope signature of the sedimentary phases in the water column, related to biological productivity. However, I do not think this conclusion is well supported by the data, finding the arguments towards this conclusion overly complicated and confusing. I therefore recommend major revisions before this paper can be accepted for publication.
Major comments
-The concept of reversible scavenging seems confused in its definition in the paper. This refers to the 2-way exchange of Pb between particles and the dissolved phases as they sink through the water column. It is likely dominated by ‘authigenic’ Pb pools via adsorption-desorption reactions. This is not to be confused with ‘detrital’ or ‘lithogenic’ Pb – Pb that is bound in mineral dust that does not ever get released to the dissolved pool, but simply sinks to be deposited in the sediment. I my view ‘scavenging’ refers to the transfer of dissolved Pb to particles (i.e. authigenic Pb), so that lithogenic Pb plays no part in ‘scavenging’ or ‘reversible scavenging’. These ideas are confused in the discussion, to create overly complicated interpretations.
-Based on the above logic, the alteration of the isotope composition of sedimentary phases by water column processes is exclusively reserved for the ‘authigenic’ Pb fraction. Furthermore, for this to occur via reversible scavenging in the water column would require some vertical gradient in dissolved Pb isotope composition – i.e. particles acquire a particular authigenic Pb isotope composition by scavenging dissolved Pb in the surface layer, which is then modified by exchanging with dissolved Pb of a different composition in the deep layer. There is no evidence put forward to suggest that this is the likely scenario to explain the data, hence I do not agree that the interpretation of the data requires the incorporation of reversible scavenging or ‘water column alteration’.
-The most likely interpretation in my view is that the Pb isotope variations reflect mixing between a lithogenic Pb pool (i.e. Pb in mineral dust that never became solubilized) and an authigenic pool (i.e. Pb that has been scavenged from the dissolved pool). This interpretation does not require any alteration of the authigenic component as claimed, only that it is over printed by lithogenic contributions to variable degrees in the bulk Pb measurements. This idea is already included in the paper, but it is confusingly linked to ideas regarding scavenging altering isotope compositions.
I would therefore like to see ideas about this data reflecting ‘water column alteration’ or being evidence of reversible scavenging being removed (change of title, reframing of introduction, and complete re-write of the results and discussion). I would also like to see the elemental data used to constrain more quantitatively the mixing between lithogenic vs detrital Pb sources suing Pb/Al and/or Pb/Ti ratios, with this analysis done early on in the discussion to establish the basic framework within to interpret the isotope data.
Minor comments
Section 2.2. Please explain how instrumental mass bias was monitored and corrected. Also what statistic is used for the quoted ‘average relative uncertainty’?
Section 2.3. How is the LOQ defined? 10 x SD of the blanks?
Section 3.1. The authors claim that the overly water column features natural dissolved Pb distributions, based on the Pb concentration of waters much further north. I don’t think this is well supported due to (1) the differences in oceanographic region of the study areas considered, (2) 16.7 pmol/kg is not necessarily the natural baseline. We have no idea what the natural Pb concentration in the ocean is, but it is likely much lower than this in my view. More importantly, what are the Pb isotope compositions of these waters? Do they conclusively fingerprint natural Pb sources rather than anthropogenic ones?
Citation: https://doi.org/10.5194/egusphere-2026-3608-RC2
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- 1
This manuscript presents Pb concentrations and isotope ratios from three sediment cores collected near South Georgia and the South Sandwich Trench and explores their relationships with sedimentary Si and organic carbon. The potential influence of biological productivity and particle cycling on Pb isotope signals is an interesting and important topic. The dataset is valuable, and the manuscript has the potential to make a useful contribution to understanding Pb cycling in the Southern Ocean.
However, the central interpretation that primary productivity and particle transformations in the water column alter the Pb isotope signatures ultimately preserved in deep-sea sediments is currently not sufficiently constrained by the bulk-sediment measurements. In particular, the contributions of lithogenic, authigenic and biologically associated Pb have not been separated, while total sedimentary Si is treated as a direct proxy for biogenic silica and productivity. The manuscript also does not adequately compare its results with existing authigenic Pb, Nd isotope and opal-flux records from the same general region.
I therefore recommend major revision. The study is potentially publishable, but the authors should more carefully distinguish observations from interpretations, consider alternative explanations and substantially qualify some of their mechanistic conclusions.
Major comments
Pb isotope compositions of bulk sediment and the identification of Pb carriers
The Pb isotope measurements were performed on total digestions of bulk sediment. Consequently, the measured Pb represents an unknown mixture of lithogenic Pb, authigenic or seawater-derived Pb, Pb associated with biogenic particles, and potentially anthropogenic Pb. Changes in the relative proportions of these components could produce the observed downcore Pb isotope variations without requiring alteration of Pb isotope signatures during particle sinking.
The authors should clarify what information can and cannot be obtained from bulk-sediment Pb isotope measurements. In particular, the manuscript should distinguish among e.g., mixing of isotopically distinct sedimentary Pb components; redistribution or isotope exchange between dissolved and particulate Pb pools; selective dissolution and scavenging. These processes are sometimes discussed interchangeably, although they have different implications. Because mass-dependent Pb isotope fractionation has not been demonstrated by the present data, terms such as “fractionation” and “alteration” should be used cautiously. In many places, “redistribution,” “isotope exchange,” “selective scavenging,” or “mixing of distinct Pb pools” would be more appropriate.
Why was biogenic silica not measured?
The interpretation of Si as a productivity proxy is central to the manuscript, but the authors measured total Si in bulk sediment. Total Si includes both biogenic opal and lithogenic silicates or quartz. Indeed, the authors acknowledge that Si may be partly lithogenic.
Why was biogenic silica or opal not measured directly? Without separating biogenic and lithogenic Si, correlations between Pb isotope ratios and total Si cannot be uniquely interpreted as evidence for a productivity control. Such correlations could also result from variations in lithogenic input, sediment mixing or dilution by biogenic material.
Ideally, the authors should provide direct opal measurements. If this is not possible, they should at least: estimate excess or biogenic Si using Al, Ti or another lithogenic indicator Sedimentary Si and TOC should also not be described as direct measures of primary production because they are additionally affected by export efficiency, preservation, dissolution, dilution and sediment focusing.
Comparison with nearby authigenic Pb isotope records
I suggest the authors to compare their results with the nearby Southern Ocean sediment record of Huang et al. (2021): Huang et al. (2021) extracted authigenic Pb isotope signatures, whereas the present study measured bulk sediment. Importantly, both studies appear to indicate only a limited contribution from anthropogenic Pb. The consistency between the two records should be discussed because it may strengthen the authors’ conclusion that anthropogenic contamination is minor in this sector of the Southern Ocean.
At the same time, the different analytical phases provide an opportunity to evaluate the origin and carrier of Pb. The authors should compare the isotope ranges of the bulk and authigenic records and discuss whether their similarity or difference can constrain the relative contributions of: lithogenic Pb derived from Patagonian dust; authigenic Pb derived from seawater; and Pb scavenged or transported by biogenic particles.
One of the principal findings of the manuscript is the relationship between Pb isotope ratios and productivity-related parameters. A highly relevant comparison is provided by Huang et al. (2020). In that study, glacial–interglacial Pb isotope variations show a close relationship with opal flux, whereas they do not follow the authigenic seawater Nd isotope record, which reflects changes in water-mass provenance. This comparison is highly relevant to the present manuscript because it supports the possibility that sedimentary Pb isotope variability is related to biological particle export rather than being controlled solely by changes in water masses.
Additional comments
Lines 92–93: Revise “as assumed by (Cheng and Hu, 2010)” to “as assumed by Cheng and Hu (2010).”
Lines 255–290: The three sites do not constitute a controlled water-depth transect because they also differ in geographic location, productivity, sedimentation rate, lithogenic input and oceanographic setting. Differences among the cores should therefore not be attributed primarily to water depth or pressure.
Lines 269–273: Higher Pb and Al concentrations at St15 may reflect stronger lithogenic input or weaker dilution by biogenic material, rather than enrichment of Pb during particle sinking. Please consider these alternatives.
Lines 273–278: The proposed ∼1 cm offset between Si and Pb/TOC is unlikely to be explained simply by different particle sinking velocities. An offset in sediment depth represents a much longer timescale than typical water-column sinking differences. Sediment mixing, chronological uncertainty, sampling resolution and post-depositional processes should be considered.
Lines 370–375 and 390–394: The proposed pressure-induced dissolution of Pb-bearing mineral phases and biogenic silica requires stronger evidence. Otherwise, it should be clearly identified as a hypothesis.
Lines 402–407: The assumption that nearly all mineral-associated Pb was dissolved during sinking is not demonstrated by the available data and should be removed or substantially qualified.
The correlation analyses are based on a relatively small number of ordered downcore samples. Because common depth trends and autocorrelation may inflate statistical significance, the authors should test whether the correlations remain after detrending or using first differences.
Lines 489–495: The seawater data used for comparison were obtained substantially farther north and may not represent seawater directly overlying the sediment sites. This limitation should be acknowledged.
Line 492: The reported sedimentary 206Pb207Pb range of 1.12–1.21 appears inconsistent with the range of 1.195–1.212 reported earlier. Please check this value.
The conclusion should more consistently use wording such as “consistent with” or “suggests,” rather than presenting selective scavenging and water-column modification as uniquely demonstrated mechanisms.
Please check the use of “absorb” and “adsorb” throughout the manuscript. For surface binding to particles, “adsorb” is generally appropriate.
Reference:
Huang, H., Gutjahr, M., Eisenhauer, A., and Kuhn, G. (2020). No detectable Weddell Sea Antarctic Bottom Water export during the Last and Penultimate Glacial Maximum. Nature Communications, 11, 1–10.
Huang, H., Gutjahr, M., Kuhn, G., Hathorne, E. C., and Eisenhauer, A. (2021). Efficient extraction of past seawater Pb and Nd isotope signatures from Southern Ocean sediments. Geochemistry, Geophysics, Geosystems, 22, e2020GC009287.