| 15 Dec 2025
Timing of the anthropogenic carbon invasion in the Southern California Current
Abstract. The role of Eastern Boundary Upwelling Systems (EBUS), such as the southern California Current System, is a well-known high productivity region driven by alongshore winds, although their role as atmospheric carbon sources and sinks is poorly understood in the global carbon cycle. In the southern CCS, off Baja California, wind-driven vertical mixing upwells nutrient and carbon-rich waters from late winter to early summer, while weaker winds during the rest of the year allow the intrusion of nutrients and carbon-depleted subtropical surface waters. Here, we interpret the isotopic composition of organic carbon and calcitic records spanning 150 years from high-resolution sediment cores collected off Baja California in the context of seasonal variability observed between 1990 and 2011. The results show a clear trend toward lighter carbon isotopic compositions of the organic and inorganic carbon for the past seven decades. These trends are similar to the atmospheric records associated with the Suess effect, suggesting an atmospheric carbon invasion into the surface waters of the California Current. Nevertheless, the slope of the atmospheric carbon isotopic records is steeper than our marine record, most likely related to the upward mixing of subsurface waters with a relatively heavier carbon isotopic signature and advection processes inherent to the strong seasonality of the CCS southern boundary. These results will allow a better characterization of the relative role of the EBUS regions in the global carbon cycle.
"Timing of the anthropogenic carbon invasion in the Southern California Current"
In their manuscript "Timing of the anthropogenic carbon invasion in the Southern California Current," Contreras-Pacheco et al. (2025) address a critical gap in our understanding of the global carbon cycle: quantifying how and when anthropogenic CO2 has penetrated the surface waters of Eastern Boundary Upwelling Systems, regions that remain poorly constrained in global carbon budgets despite playing a disproportionately important role as seasonal sources and sinks of atmospheric carbon. The authors address this gap through stable carbon isotope records spanning approximately 150 years, derived from organic carbon and shells from two foraminifera species in laminated sediments. These sediments offer an unusual high resolution archive for the southern domain of the California current, a region that is at the boundary between cool productive waters and warm subtropical waters and therefore sensitive to changes in both upwelling intensity and atmospheric carbon loading.
The manuscript has several strengths, such as the well-justified study site and the use of laminated sediments that provide a high-resolution archive extending back to the preindustrial era. The utilization of the multi-proxy approach combining two planktonic foraminifera from different depth habitats is methodologically sound. The integration of modern SOCAT CO2 fugacity data to establish the seasonal source-sink behavior of the region before interpreting the long-term sedimentary record is particularly commendable, as it grounds the paleoceanographic findings in directly observable present-day dynamics.
However, I have a few comments that I think will make the manuscript more robust. My main concerns revolve around quantitative analysis, lack of statistical treatment, and uncertainty quantification.
1) My first and most important concern relates to the chronological framework of the study. For a manuscript whose title explicitly addresses the timing of anthropogenic carbon invasion, the age model is surprisingly underdeveloped and, as presented, cannot be independently evaluated by the reader. In the main text, the age model receives only a single short paragraph (Section 2.4), with no figure, no sedimentation rates, and no uncertainty quantification. The supplementary material, while providing Figure S1, does little to resolve this concern. The entire chronology of core SaLa11-E19-MC1 rests on a single-proxy stratigraphic correlation based on calcium carbonate content matched against core BAP96-6C. This approach raises several methodological concerns because the visual correlation between the two carbonates is ambiguous, multiple peaks could be matched in different ways, resulting in a potentially non-unique stratigraphic alignment. There is also the underlying assumption that carbonate fluctuations are synchronous across the basin, which is stated, but it is not demonstrated, which makes the reader incapable of evaluating the reliability of the chronology on which everything else depends. The age uncertainty is never quantified. I suggest explicitly mark and justify the tie points used in the carbonate correlation, quantify the age uncertainty propagated into SaLa11-E19-MC1, and discuss how the 15-year gap between core collection dates (1996 versus 2011) was handled in the stratigraphic alignment.
2) A second important gap concerns the statistical treatment of the isotopic trends, which are the paper's primary quantitative contribution. The authors present slope comparisons between atmospheric and marine δ¹³C records reporting values of −0.12‰ per decade for foraminiferal calcite and −0.15‰ per decade for organic carbon, against −0.27‰ per decade for the atmosphere as central findings of the study, yet none of these trends are accompanied by any formal statistical analysis. There are no confidence intervals, no R2 values, no p-values, and no uncertainty propagation reported anywhere in the manuscript, given the substantial variability evident in Figures 3–5. Without knowing whether these slopes are statistically distinguishable from one another, or whether the trends themselves are statistically significant, the paper's main interpretive argument that the marine records show a systematically lower slope than the atmospheric trend due to upwelling buffering cannot be evaluated. I suggest a test of significance to determine whether the marine slopes are distinguishable from the atmospheric slopes.
3) Another aspect I am concerned about relates to the assumption of a constant phytoplankton fractionation factor used to reconstruct δ¹³C_DIC from the organic carbon record. The authors derive a fixed value of ε = −21‰ from the preindustrial period (1800–1940) and apply it uniformly across the entire 150-year record. The authors cite Young et al. (2013) whom in their paper, showed that phytoplankton fractionation change measurably between 1960 and 2010 in response to rising atmospheric CO2, using a fixed preindustrial fractionation factor to the post 1950 introduces a systematic bias into the reconstructed δ¹³C_DIC values which are then used to validate the foraminiferal calcite in Figure 5. The authors should explicitly acknowledge this limitation in the manuscript.
4) In Figure 5, the authors shows the presence of an δ¹³C offset between G. ruber and N. dutertrei explained in the text purely in terms of the different depth habitats of the species but do not discuss how the depths of the calcification may shift seasonally and interannually in response to changes in upwelling intensity, thermocline depth and chlorophyll maximum position especially for N. dutertrei whose preferred habitat is tied to the chlorophyll max. if the habitats have changed over the 150 year record, in response for example to the upwelling intensity this could introduce a non atmospheric component into the isotopic trends that are currently attributed entirely to the Suess effect, the author should address the potential sources of bias explicitly in the manuscript.
4) I also have a few comments related to the structure and depth of the discussion section, which is largely qualitative and somewhat circular. In the paper, the author shows that the upwelling of older, isotopically heavier subsurface waters partially buffers the atmospheric Suess effect signal, explaining the shallower slope observed in the marine records relative to the atmosphere. This is invoked repeatedly but never quantitatively, is it possible to estimate what fraction of the observed slope attenuation can be explained by subsurface water mixing alone?
5) I think it would be better to compare the other EBUS records from different ocean basins in the discussion such as Humboldt Current off Peru and Chile, or the Benguela Current off southwest Africa. This would substantially strengthen the paper's contribution to understanding how the Suess effect propagates through EBUS regions globally. If similar slope attenuations are observed in other EBUS sedimentary records, this would powerfully support the authors' mechanistic interpretation and elevate the paper's findings from a regional observation to a globally relevant result. However, if the attenuation observed in their system is stronger or weaker than in other EBUS regions, this would open an important discussion about how additional factors can control the regional expression of the Suess effect. I suggest that the authors add a dedicated paragraph in the discussion addressing this broader EBUS context.
Minor comments:
1) Abstract: Line 12 should mention the abbreviation CCS used in line 14
2) Lines 12-13 needs to be rewritten. How is the role of EBUS a well known high productivity region? Maybe remove ‘the role of’
3) Lines 28-29 need to be rewritten or removed. it is better to start the introduction by line 32: ‘over the past century, the ocean’s carbon uptake… then talk about the processes.
4) lines 37-40: There is no obvious link between this part and the previous part.
5) The caption for Figure 1 appears to be duplicated in the text at lines 117–119 of the manuscript.
6) Figure 3 is particularly overcrowded, with multiple cores, proxies, and reference datasets sharing overlapping symbol styles. I suggest at least separating the 3 figures so the separation is visual.
7) The conclusions section introduces some new interpretive content that does not appear in the discussion, particularly regarding the 1950s shift mirroring the Law Dome ice core record, which should either be developed properly in the discussion, or removed from the conclusions or mentioned in some way as a perspective.