Widely used to infer past climates, isotope measurements from stalactites and stalagmites in caves can mislead researchers.
They are among the most useful storytellers of earth history: speleothems, or cave formations. Scientists collect samples from stalactites and stalagmites, take them to their labs, and measure the fractions of stable oxygen and carbon isotopes found in their inclusions of crystals of calcium carbonate or calcite. From these crystal balls, the scientists look far into the past, envisioning climate change and long ages. What could be more straightforward? The data prove it.
A funny thing happens on the way to the lab. The isotopic fractionations become altered. Here’s what a team of speleologists (cave scientists) conclude from expeditions into a couple of caves in Hungary.
Speleothem deposits are among the most valuable continental formations in paleoclimate research, as they can be dated using absolute dating methods, and they also provide valuable climate proxies. However, alteration processes such as post-depositional mineralogical transformations can significantly influence the paleoclimatic application of their geochemical data.
Climate scientists know about some of the alteration processes, but this team points out new ones that have not been appreciated. The paper in Nature‘s open-access journal Scientific Reports concentrates on one alteration process—the transformation of amorphous calcium carbonate (ACC) to nanocrystalline calcite—but suggests there are other factors that, if not properly accounted for, can have “serious consequences” on the interpretation given the data.
Here’s the basic problem with ACC: it depletes the fraction of stable oxygen-18 ions (δ18O) before transforming into crystalline calcite. This happens within a few hours or days, potentially giving wrong readings in the lab when the speleothems are measured. The scientists, therefore, might be measuring different values in their refrigerator samples than the crystal as it was forming in the cave long ago. If they expect their measurements to represent a “proxy” (a measurement standing in for something else, like climate), they could be fooling themselves.
Detection of ACC is rather difficult in cave deposits, as ACC can undergo transformation to calcite in minutes in a hydrous environment, and even stabilizing compounds like Mg or organic matter are only capable of extending its stability to some weeks. Taking into consideration the general precipitation rate (0.1 to 1 mm per year), the collection of carbonate in appropriate amounts for mineralogical or geochemical analyses requires several months. Over the course of such a long collection time, however, the original ACC can be transformed into calcite. Although ACC preparation in the laboratory is a routine procedure, its synthesis requires conditions distinctly different from those to be found in natural cave environments, e.g. mixing of CaCl2 and NaCO3 or (NH4)2CO3 solutions. Hence, the preparation conditions and characteristics of synthetic ACC render it inappropriate to function as an analogue of its natural counterpart, thus it cannot provide the information sought.
The scientists observed ACC forming onsite in the cave on special collection surfaces. The ACC can exist in open or closed systems, depending on whether the inclusions become embedded within the dripwater, reaching equilibrium. The researchers in the lab will not always be able to tell whether calcite from which they obtain δ18O measurements reflect actual conditions in the cave or altered conditions when the ACC lowered the value during crystallization. The conclusions on which they base dates or paleoclimates could be in error.
If this were the only worry, perhaps scientists could learn to correct for it by identifying other proxies for the presence of ACC. Unfortunately, this is not the only concern. ACC formation is a function of temperature, conductivity, pH, CO2 concentration, degassing rate, evaporation rate, drip rate and other factors. Unless these factors are known and controlled, and unless researchers gather their data in actual cave environments, they could be misled.
The team also notes that scientists get different equations whether they use theoretical analyses, experimental techniques or empirical observations.
Uncertainties in the estimation of ACC amount is a major weakness in the fractionation calculation, hence the verification of calcite-ACC fractionation estimation requires independent information provided either by experimental studies or by natural analogues. The experimental determination of ACC-water oxygen isotope fractionation representative for speleothem formation is challenging because (i) ACC rapidly transforms to calcite during the preparation and (ii) laboratory ACC synthesis requires physical and chemical conditions distinctly different from those found in a cave environment. Available estimations of δ18O differences between crystalline and amorphous carbonates formed in natural environments suggest that the crystalline carbonate is several ‰ more enriched in 18O than its amorphous counterpart (dolomite, aragonite, Mg-calcite).
What you actually get may not be what you believe you got. Different caves and different forms of calcium carbonate may give very different results. The paper sounds a warning call to researchers:
The present study provides direct evidence for relatively 18O-depleted ACC formation in caves at about 10 °C. Since the δ18O value of inclusion-hosted water may carry significant paleoclimatic/paleohydrological information, it is important to note that its use is limited by the cave environment.
The authors add one more source of uncertainty: microbes. They toss out that potentially significant alteration right at the end of the paper, after summarizing reasons why you can’t trust the values in this “most valuable” method of inferring paleoclimates and dates. Oh, and don’t forget the unidentified organic compounds in the dripwater, which can vary significantly from cave to cave and also affect ACC formation.
A number of experimental studies have shown that the formation and stability of ACC may be influenced by the physical parameters of the ambient environment and the chemical compositions of the parent solutions. In natural cave environments the most important factors might be the cave temperature, drip water pH, as well as concentrations of Mg, SO42− and organic compounds in the solution. A comprehensive study is suggested to cover several cave environments with different temperatures, ventilation degree, soil characteristics, drip water chemistry and carbonate growth rates in order to determine the exact factors governing ACC formation. The transition from ACC to calcite has been shown to take place in several steps with intermediate hydration states and mineral phases like vaterite. Investigations on the ACC-calcite transition and its governing factors require monitoring of mineralogical changes at high temporal resolution. Additionally to the inorganic factors, the role of microbial activity should also be investigated. Amorphous carbonates are ubiquitously secreted by living organisms in sedimentary environments, hence microbial mediated carbonate precipitation is also a potentially important process in ACC formation, whose exploration requires systematic biological/biochemical investigations.
It appears that climatologists leaning on cave data know a lot less than they thought they knew. This final paragraph almost makes it seem like it’s time to toss out the equations and interpretations and start over. Isn’t that implied by “a comprehensive study is suggested” using “systematic biological/biochemical investigations” in multiple caves with varying conditions?
Reference: “Formation of amorphous calcium carbonate in caves and its implications for speleothem research” by Attila Demény, Péter Németh, et al., Scientific Reports 6, Article number: 39602 (2016), doi:10.1038/srep39602, published 22 Dec 2016.
Remember this paper the next time you are presented with scientific “facts” that prove a scientific “consensus” of one sort or another. The conclusions of any empirical study cannot be divorced from the assumptions that go into those conclusions. A consensus is most dangerous when the conclusion is decided in advance, and scientists within a preferred paradigm go out looking for evidence to confirm it.