Mapping Fluid‐Filled Inclusions in Stalagmites Using Coupled X‐Ray and Neutron Computed Tomography: Potential as a Water Excess Proxy

Abstract

Application of novel proxies, such as the stable isotope compositions and noble gas concentrations of fossil drip water trapped as inclusions in stalagmites, have the potential to provide unique constraints on past hydroclimate states and surface temperatures. Geochemical analysis of inclusion waters, however, requires an understanding of the three‐dimensional spatial distribution of dominantly liquid‐ versus air‐filled inclusions in a given stalagmite. Here we couple neutron computed tomography and medium‐ to high‐resolution X‐ray computed tomography to map out the three‐dimensional calcite density and distribution of liquid‐ versus air‐filled inclusions within a Sierra Nevada stalagmite (ML‐1), which formed during the last deglaciation (18.5 to 11.7 ka). Comparison of coupled neutron computed tomography‐X‐ray computed tomography results with a time series of stalagmite calcite fabrics indicates that although highest density calcite contains abundant liquid (fluid)‐filled inclusions, calcite density and fabric overall were secondary controls on the liquid inclusion distribution (LID). Furthermore, a multistatistical evaluation of the stalagmite time series indicates a significant relationship at the multicentury‐ to millennial‐scale between LID and calcite δ18O and δ13C that suggests a potential link between LID and water availability to the cave.

North Atlantic meltwater during Heinrich Stadial 1 drives wetter climate with more atmospheric rivers in western North America

“…This finding is consistent with the McLean's Cave proxy records that suggest both a wetter climate during HS1, as indicated by a shift to lower values in the calcite stable carbon isotope record (Fig. 1G) and inclusion fluid hydrogen and oxygen isotope records, and an increase in moisture from warm, wet storms from the subtropical Pacific, as indicated by a positive shift in the calcite stable oxygen isotope record during HS1 relative to the late glacial (23)(24)(25)(26). Shifts in speleothem carbon and oxygen isotope records between the LGM and HS1 are evident further north at Lake Shasta Caverns, California (Fig.…”

“…1). Several proxy records from California also point to a wetter HS1, including multiproxy stalagmite records from the Sierra Nevada in eastern California (24)(25)(26) and the Klamath Mountains in Northern California (27), as well as salinity proxies from Searle's Lake in the Mojave Desert (28) and sediment proxies of runoff from Lake Elsinore in southern California (29). In contrast, there is no discernable difference between LGM and HS1 lake levels in Panamint Valley (30), and soil opal uranium isotopes from Fish Lake Valley, Nevada, also do not show a clearly wetter HS1 (31).…”

“…LGM conditions are determined for 21,000 ± 1000 years B.P. for each record with the exception of the ML1 stalagmite (24)(25)(26), which extends to 19,400 years B. P. For this record, we take the late glacial portion of the carbon isotope record (18,400 to 19,400 years B.P.) to represent LGM climate at this site.…”

Detecting and quantifying palaeoseasonality in stalagmites using geochemical and modelling approaches

“…Non-destructive methods include (but are not restricted to): i) simple desktop scanning and photography, ii) µXRF line scanning and mapping (e.g., Breitenbach et al, 2019;, iii) synchrotron analyses (e.g., Frisia et al, 2005;Vanghi et al, 2019;Wang et al, 2019b;Wynn et al, 2014), iv) phosphor mapping via beta-scanning (e.g., Cole et al, 2003), v) reflected light, and fluorescence, including confocal laser fluorescent microscopy (CLFM) (e.g., Orland et al, 2012) and other microscopy techniques (e.g. SEM, EMPA, RAMAN), or vi) X-ray Computed Tomography (CT) scanning (e.g., Walczak et al, 2015;Wortham et al, 2019). The choice of technique should consider suitability for answering the targeted research questions, and logistical considerations such as sample sectioning.…”

“…Similarly, the sample should allow 2-3 mm sampling into the depth of the sample slab, and ideally the growth layers should not taper out in the third dimension. X-ray and Neutron CT scans can help visualise the 3D internal structure of the sample (Walczak et al, 2015;Wortham et al, 2019), and the appropriate milling depth.…”