the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Distinguishing the vegetation and soil component of δ13C variation in speleothem records from degassing and prior calcite precipitation effects
Heather M. Stoll
Chris Day
Franziska Lechleitner
Oliver Kost
Laura Endres
Jakub Sliwinski
Carlos Pérez-Mejías
Hai Cheng
Denis Scholz
Abstract. The carbon isotopic signature inherited from soil/epikarst processes may be modified by degassing and prior calcite precipitation (PCP) before its imprint on speleothem calcite. Despite laboratory demonstration of PCP effects on carbon isotopes and increasingly sophisticated models of the governing processes, to date, there has been limited effort to deconvolve the dual PCP and soil/epikarst components in measured speleothem isotopic time series. In this contribution, we explore the feasibility, advantages, and disadvantages of using trace element ratios and δ44Ca to remove the overprinting effect of PCP on measured δ13C to infer the temporal variations in the initial δ13C of dripwater. In 8 examined stalagmites, the most widely utilized PCP indicators Mg/Ca and δ44Ca covary as expected. However, Sr/Ca does not show consistent relationships with δ44Ca so PCP is not universally the dominant control on Sr/Ca. From δ44Ca and Mg/Ca, our calculation of PCP as fCa, fraction of initial Ca remaining at the deposition of the stalagmite layer, yields multiple viable solutions depending on the assumed δ44Ca fractionation factor and inferred variation in DMg. Uncertainty in the effective fractionation of δ13C during degassing and precipitation contributes to uncertainty in the absolute value of estimated initial δ13C. Nonetheless, the trends in initial δ13C are less sensitive to these uncertainties. In coeval stalagmites from the same cave spanning 94 to 82 ka interval, trends in calculated initial δ13C are more similar than those in measured δ13C, and reveal a common positive anomaly initial δ13C during a stadial cooling event. During deglaciations, the trend of greater respiration rates and higher soil CO2 is captured in the calculated initial δ13C, despite the tendency of higher interglacial dripwater situation to favor more extensive PCP.
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Heather M. Stoll et al.
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RC1: 'Comment on cp-2022-77', Anonymous Referee #1, 03 Jan 2023
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The manuscript "Distinguishing vegetation and soil component of d13C variation in speleothem records from degassing prior calcite precipitation effects" by Stoll et al. presents a high quality scientific contribution to the interpretation of speleothem proxy records, in particular carbon isotopes, as well as the trace metal ratios (Mg/Ca) and (Sr/Ca) also explored in the manuscript. It is within scope of Climate of the Past for the development of methods and understanding of speleothem proxy data, as well as some new data in the case studies. The results and interpretation well-argued and presented in an appropriately structured way in terms of text and figures/tables.
The approach is novel and the methods valid. The manuscript is particularly well-presented in its presentation of the complexity of carbon isotopes and PCP, providing a useful review of these proxies as well as a novel method towards improved quantification of proxies. The paper also well-outlines the uncertainties in the approach and then develops a method to numerically incorporate these uncertainties into the quantification of the impact of degassing/PCP on speleothem carbon isotopes. It is shown that while the uncertainties preclude a reconstruct an actual soil CO2 d13C value, it is shown that the method may still be applied to examine the impact on the trend of the reconstructed soil δ13C compared with the measured δ13C values in the speleothem. This is the strength of the paper and is well-illustrated in the case studies. I appreciated the detailed appraisal of the method and output.
As well as providing a method for correcting the PCP impact on d13C via Mg/Ca, my impression of other strengths of this paper are also is in its demonstration that speleothem DMg may vary within a speleothem time series and that this may actually be expected to be common. I think that this point is an important contribution to the community and it is well-explored in the paper, although please see point below about fabrics and flow paths. The other major finding that I think should be useful to the community is that the soil PCO2 and PCP processes can have counteracting influences on the resulting stalagmite δ13C and produce observed nearly constant δ13C timeseries during significant climatic transitions. It is demonstrated in the two case studies and provides good motivation for others to investigate their datasets with this method.
There appears to be cave monitoring paper from the same cave(?) in review (Kost et al) that this was not cited in the reference list. If it were (and if in open access review?), it would have been interesting to see whether these monitoring data support the approach presented here. Hopefully, this is covered in the Kost paper.
As outlined above, the authors have well-outlined the uncertainties in their approach, although there are two other possible processes that were not or perhaps under-acknowledged. One is the the recent findings by Frisia et al 2022 in Quaternary Science Reviews. This paper showed that Mg/Ca in the calcite may vary with fabric porosity. This will affect the partition coefficient, DMg, which feeds into the equations for correcting PCP impact on d13C. This should be included and the findings of Frisia et al paper also implies that the Stoll et al manuscript would benefit from a description of the fabrics and whether they differ within a stalagmite over sections where DMg is thought to have varied. Similarly, karst flow paths received one brief mention around line 310 yet there has been much discussion in the literature around flow paths and PCP. The manuscript should include a few more sentences around this, rather than focus on PCP has just being a function of CO2 gradients and drip rate. Finally, the fabrics and even speleothem morphology can provide a useful indication of past drip rates and this is covered in multiple other studies and in the Speleothem Science book by Fairchild & Baker. Seeing as the influence of drip rate on PCP appears to be important yet we cannot know how drip rate has varied in the past, some description of the fabrics and speleothem morphology could have been helpful as these can be an indication on whether drip rate has varied in the past.
Other points:
More correct wording of the title may be: “Distinguishing soil component of d13C variation in speleothem records from degassing prior calcite precipitation effects”. The approach is not able to specifically isolate vegetation d13C from the soil d13C CO2 pool and does not discuss vegetation-derived d13C CO2 at any length in the manuscript.
L19: the term “overprinting” is used to describe PCP on speleothem δ13C here and throughout. Suggest “contribution” would be more correct term.
L22: “universally”, this is not a global study so the conclusion can only be that PCP is not the dominant control on this particular study site. Suggest reword.
L28: this sentence is unclear and needs rewording, suggest: “During glaciations, calculated initial δ13C implies trend of increasing respiration rates and higher soil CO2, despite the interpreted reduced drip flux to favour more extensive PCP”?
L32: Why not the lower latitudes? Doesn’t Figure 4a contradict this introductory sentence?
L39: root depth and soil moisture content also contribute.
Table 1: looks like a copy and paste mistake for scenario A1 and A2 as identical definitions given for these variables. This made it a little hard unfortunately to understand some of the distinction in the modelled results for these two scenarios although I don’t think was an important factor in being able to follow the main findings of the paper. But needs fixing here. Also, Delta Ca should be Delta44Ca.
L167: suggest precipitation rate rather than growth rate dependent, i.e., the loss of calcium is not necessarily related to growth rate. Growth rate is usually used to define the linear extension rate in the vertical axis of a stalagmite.
Sub-heading 2.2.3. Are there a few more papers on D44Ca and PCP worth citing here?
L216-217: is this statement supported by the back modelling in the results?
L222: please add the mass spectrometer specifications.
L231: please reword as it is unclear as to whether there were five repeats on each aliquot or whether there were five analyses performed on each stalagmite?
L240: preferred value rather than preferred measurement?
L253: please refer back to equation 3 here.
L300: La Vallina Cave?
L300 paragraph starting here becomes confusing as total range and “fold” range not the same thing. Confusion ensues here for several paragraphs until the reader sees the definition of range used is the maximum/min rather than max minus min value. This definition does not come until the caption in the following table. Please define in the text to assist the reader.
Fig 4 caption: for soil moisture which direction was the experiment? Do the green crosses indicate increasing or decreasing soil moisture?
L432: there should be some earlier description of these dripwater measurements and their method, seeing as the Kost et al paper is not yet published.
Reference:
Silvia Frisia, Andrea Borsato, Adam Hartland, Mohammadali Faraji, Attila Demeny, Russell N. Drysdale, Christopher E. Marjo, Crystallization pathways, fabrics and the capture of climate proxies in speleothems: Examples from the tropics, Quaternary Science Reviews, Volume 297, 2022, https://doi.org/10.1016/j.quascirev.2022.107833.
Citation: https://doi.org/10.5194/cp-2022-77-RC1
Heather M. Stoll et al.
Heather M. Stoll et al.
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