the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jun 2025
Monthly element/Ca trends and inter chamber variability in two planktic Foraminifera species: Globigerinoides ruber albus and Turborotalita clarkei from a hypersaline oligotrophic sea
Abstract. Environmental and biological factors influence the trace element composition (element/Ca) of planktic foraminifer shells. Consequently, the element/Ca measured in these shells (tests) are utilized as proxies to reconstruct past oceanic and climatic conditions. As single shell analyses are increasingly used in paleoceanographic research it is important to understand how proxy systematics change between species, individuals of the same species in a given population, and among chambers of a single individual during its life cycle. Here we present a time series of the chemical composition of planktic foraminifers retrieved using sediment traps between June 2014 and June 2015 at the northern part of the Gulf of Aqaba (aka Gulf of Eilat). Laser ablation ICP-MS element/Ca measurements were performed on single shells and chambers of Globigerinoides ruber albus and Turborotalita clarkei, collected monthly from five water depths (120 m, 220 m, 350 m, 450 m, and 570 m). Sediment trap samples were paired with corresponding data on water column hydrography and chemistry. Pooled means of measured element/Ca display species-specific and element-specific behavior, with generally higher values for T. clarkei phenotypes (‘big’ and ‘encrusted’) in comparison to G. ruber albus. Some element/Ca values measured in water column specimens, such as Al/Ca, vary significantly from core-top specimens. A unique finding is a prominent increase in element/Ca around March–April 2015, during maximum water column mixing, mostly apparent in T. clarkei and to a lesser extent in G. ruber albus. This spring element/Ca increase is observed in most measured elements and is further associated with an increase in inter-chamber variability (ICV). Inter-chamber element/Ca patterns show element enrichment/depletion in the most recently precipitated (final, F0) chamber in comparison to the older chambers (penultimate (F-1), antepenultimate (F-2), etc.). Element/Ca in F0 may also be less sensitive to surrounding environmental conditions. For example, the Mg/Ca of the F-1 and F-2 chambers of G. ruber albus display a positive relationship with mixed layer temperatures while F0 does not. To overcome this effect, we suggest using pooled means from non-F0 fractions as environmental records and paleo proxies.
These results highlight the complexity of proxy systematics that rises from the variability in element/Ca measured among different species and between chambers, caused by ecological conditions and other processes in the water column including physical, chemical, and biological effects.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1929_De Nooijer', Lennart de Nooijer, 30 Jun 2025
Dear editor,
The manuscript you asked me to review (egusphere-2025-1929) by Levy and co-authors is full of interesting data and could prove to be a useful contribution within our field. It sheds light on the variability, temporal and between species, of El/Ca applied in paleoceanography. However, the quality of the manuscript could be greatly enhanced by restructuring the Results section. It appeared to me that the potential of this dataset is far greater than it now is used. This is also reflected by an unclear rationale (end of the Introduction): what exactly do the authors aim to show with this dataset? Therefore, I recommend major revisions: below, I listed my major concerns, and the attached pdf has some more, minor suggestions for improvement.
In summary: not just the means, but the full single-chamber El/Ca should be shown and (statistically) analyzed. Now, only the standard error is shown (figure 3, although very difficult to distinguish). There are multiple questions that the authors could answer: what exactly is the between-chamber variability in El/Ca and how does this relate to the chamber number? Does that change with time? Is it similar between depths and is it similar for the different elements? If there are differences, are they significant?
This will also require a full report on some basic metrics: how many specimens and how many chambers were analyzed? What was the variability within ablation profiles? Etc. The totality of data that the authors have is impressive (both for the foraminifera and the water column characteristics) and could provide much more insights than presented now.
Much of the current Results is spent on differences in time for each of the water depth. But the patterns are very similar, so instead of repeating the results for the different water depths, I suggest to systematically answer the type of/ some of the questions I listed above and illustrate those with new figures.
Including the MLD in figures 4-8 is confusing, at least in this way. It is the same for every panel. Maybe it works to include it as a color for when a sediment trap is above, and another color for when it is below the MLD. Hope I am making myself clear: the two colors would alternate within a panel and also be different for the different depths (bur obviously remain the same for the three taxa. It may even be sufficient to include that information just for G. ruber.
There is a surprising lack of statistical analysis, while the data allows for comparison along all kinds of dimensions (species, chambers, depths, etc.), which I therefore strongly encourage. The Spearman correlation matrix (figure 9, where the elements should not be near the tick marks between the squares, btw) may not be very useful here: the preceding figures show that the behavior between element in the F-chamber, for example, is very similar. I find it interesting that on that level, some of the elements behave very similar (e.g. Mg and Sr), which is lost in the larger comparison of the correlation matrix. To disentangle the effect of the different parameters (species, depth, core top or trap, time, MLD, etc.) on the El/Ca and similarity between elements, an RDA may be more appropriate. This would also require rearrangement of section 4.2.
The global compilation (section 5.3) is out of place. Here, all kinds of species are lumped, as well as types of analysis, seasons, etc. It takes a whole other approach to summarize this data and look for meaningful patterns. In the current version of this manuscript, it is also not clear what the overall goal of this comparison is and therefore it is not logically related to the Results and the rest of the Discussion.
Sincerely,
Lennart de Nooijer
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RC2: 'Comment on egusphere-2025-1929', Takashi Toyofuku, 12 Aug 2025
In this manuscript, the authors, through single-chamber analysis (LA-ICP-MS) of planktonic foraminifera in the high-salinity and oligotrophic environment (GOA: Gulf of Aqaba), captured in detail the temporal and vertical variations of element/Ca ratios. Especially, the thorough monthly sampling at each depth is considered by this reviewer to make this work quite unique. Also, by the analysis of each single chamber, the authors focus on inter-chamber variability (ICV) and propose it as an event proxy, which is also characteristic.
In the monthly time-series analysis, they capture the rapid chemical composition change associated with the event of spring mixing.
In the study subjects (G. ruber albus and T. clarkei “big” / “encrusted”), the difference of characteristics for each is shown, and the importance of species-dependency in proxy use is indicated. These efforts can become the basis to establish the chamber-by-chamber record of planktonic foraminifera as a future high-temporal-resolution paleoenvironmental reconstruction method.
The expected audience is as follows, and this fits with the scope of the journal:
Paleoceanography and paleoclimatology researchers interested in high-resolution records and short-term event analysis
Geochemistry and biomineralization researchers who want to understand element uptake mechanism and calcification physiology
Analytical chemistry and LA-ICP-MS application researchers who want to know possibilities and limitations of micro-scale analysis
Before publishing, I would like to comment the points which I noticed should be solved:
1. Na/Ca spikes
The reason for Na/Ca spike cannot be explained by the increase of Na concentration in seawater. Foraminiferal Na/Ca is also an indicator of salinity. If we try to explain the large Na/Ca variation as in Fig. 7 by seawater Na/Ca variation, we have to suppose an event in which salinity increases by two digits scale. However, from the desert in the hinterland, even if minerals are input and somewhat dissolved, it is hard to consider an impact on the salinity at the scale of digit change. Also, from the analysis of T. clarkei in Fig. 9b, c, Na shows positive correlation with Mg, Al, Ti, Mn, Fe. This may suggest that T. clarkei has the property to incorporate sinking particles containing Na, such as Albite/Na-feldspar and Plagioclase, on/into the shell during calcification. I guess the possibility of particle trap on the shell. This is possibly indicating a new role of foraminifera as “fossils that trapped sinking particles” in addition to being environmental proxies. From Fig. 7(b), there is a possibility that the same phenomenon is happening in G. ruber. If this is because the study area has an arid region in the background and seasonally sinking particles become extremely abundant in seawater, this does not affect the soundness of proxies using planktonic foraminifera from other regions. Also, if we can monitor some elements like Ti, Na or Si to check whether the proxy is working normally, the soundness of the environmental proxy in the study area is also kept, while the role as a catcher of sinking particles itself will emerge.
From the perspective of calcification mechanisms, the calcifying fluid is to some extent isolated from ambient seawater, making the direct incorporation of external particles unlikely. However, it is possible that particles adhering to the shell surface become enclosed when a new chamber is formed over them. Although unpublished, in my own experience this reviewer has observed cases where diatom frustules were incorporated into the interior of the shell. In other words, the incorporation of foreign material into the shell interior can indeed occur. While such occurrences have generally been rare enough to go unnoticed, this study might find by the possibility that in certain seasons in this particular region, such incorporation might happen more frequently.
To verify whether the Na/Ca spikes originate from calcite itself or from the incorporation of external mineral particles, it is necessary to conduct some form of direct check. For example, confirming the amount and seasonal changes of sinking particles in the study area, and performing SEM observations or XRD analysis of the shells, would allow you to determine whether mineral-like foreign materials are present inside the calcite. Alternatively, by examining the depth-resolved elemental profiles obtained from the authors’ LA-ICP-MS analyses, it should be possible to determine whether the influence of external particles extends throughout the entire calcite structure or is confined to specific locations.
Establishing this point is essential for assessing the reliability of Na/Ca as a proxy in this environment, and solving it would also strengthen the discussion of Fig. 12.
2. Final chamber (F0) composition
Regarding the idea that F0 (final chamber) element composition does not reflect the environment, I think there are both opinions, but to deny it here needs a little more basis. For example, Sadekov et al. (2009: https://doi.org/10.1029/2008PA001664) concluded that Mg/Ca of the final chamber has the highest correlation with temperature, and Hupp and Fehrenbacher (2024: https://doi.org/10.61551/gsjfr.54.4.355) also did not point out problems in analyzing the final chamber. There are other similar studies. Especially Mg/Ca has many cases that respond rather straightforward to temperature changes, so I would be more convinced if you point out that calcification temperature (depth) is different from the assumption.
3. Small number of individuals
I appreciate again the accumulation of efforts that you analyze three categories of foraminifera at each depth every month, which is very ambitious. However, the small number of individuals in each population is obvious. ICV is discussed, but I do not find quantitative treatment of inter-individual variability or pooled mean value. As the basis to say that discussion is possible with few individuals, could you add, in addition to pooled mean value, statistical indices showing the magnitude of variation among individuals (for example: standard deviation, coefficient of variation) or excuses from previous studies which state that comparing by pooled mean value for inter-individual variation is no problem?
4. Minor points
Fig. 2 appears quite late in the text. You forget to refer to Fig. 2 somewhere in the first half.
In the text final chamber is written as F0, but in Fig. 1 it is F-0. Please unify.
In Fig. 4 etc., please indicate MLD also in the legend.
Comparing the environmental figure in Fig. 1 with Fig. 3–8, the horizontal axis is shifted. Please fix, and please also write what the horizontal axis represents.
In Fig. 9, elements are slightly misaligned with the columns. For example, U is completely showing the result of the previous element for both vertical and horizontal axes.
Citation: https://doi.org/10.5194/egusphere-2025-1929-RC2
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