Local hydroclimate alters interpretation of speleothem δ18O records

Abstract

Oxygen isotopes (δ¹⁸O) are the most commonly utilized speleothem proxy and have provided many foundational records of paleoclimate. Thus, understanding processes affecting speleothem δ¹⁸O is crucial. Yet, prior calcite precipitation (PCP), a process driven by local hydrology, is a widely ignored control of speleothem δ¹⁸O. Here we investigate the effects of PCP on a stalagmite δ¹⁸O record from central Vietnam, spanning 45 - 4 ka. We employ a geochemical model that utilizes speleothem Mg/Ca and cave monitoring data to correct the δ¹⁸O record for PCP effects. The resulting record exhibits improved agreement with regional speleothem δ¹⁸O records and climate model simulations, suggesting that the corrected record more accurately reflects precipitation δ¹⁸O (δ¹⁸O_p). Without considering PCP, our interpretations of the δ¹⁸O record would have been misleading. To avoid misinterpretations of speleothem δ¹⁸O, our results emphasize the necessity of considering PCP as a significant driver of speleothem δ¹⁸O.

Fig. 1. Schematic of prior calcite precipitation (PCP) in a cave system.

The left panel shows the flow path of drip water and calcite precipitation along the infiltration pathway. Green arrows denote CO₂ fluxes, and blue dashed lines denote water flow. The right panel shows detailed changes in isotopes (δ¹⁸O and δ¹³C) and trace elements (Mg/Ca) during PCP in (a) the epikarst and (b) the cave ceiling.

Fig. 2. Modern and pre-industrial iTRACE climatology.

a Map of total annual precipitation in Mainland Southeast Asia derived from Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation (APHRODITE). Arrows show wind direction of the SW monsoon and NE monsoon with average δ¹⁸O_p values of JJA and SON precipitation. The orange square denotes the study site location. The average seasonal cycle of (b) rainfall from APHRODITE (dark blue) and iTRACE Pre-industrial simulations (light blue) and (c) δ¹⁸O_p measured from rainfall collected at Phong Nha-Ke Bang Park Headquarters (orange) and from iTRACE Pre-industrial simulations (red). APHRODITE and iTRACE time series taken from grid cell encompassing the study site.

Fig. 3. Stalagmite (HH-1) proxies from 45 – 4 ka.

a HH-1 δ¹⁸O (orange) and corrected δ¹⁸O (δ¹⁸O_corr, black). The black line is the correction using the best choice parameter configuration (see Supplementary Table S1) and α_{calcite-water} from ref. (see “Methods”). The gray shading denotes the standard deviation of the 1000 Monte Carlo simulations from the sensitivity testing. b HH-1 Mg/Ca (red) and δ¹³C (pink). The black dashed line shows the Mg/Ca threshold used to determine Mg/Ca_i. Beige shading denotes the Younger Dryas and Heinrich Stadials 1–4. The Mg/Ca, δ¹³C, and δ¹⁸O curves are from ref. .

Fig. 4. HH-1 compared to Chinese stalagmites.

a Time series of HH-1 δ¹⁸O (orange), HH-1 δ¹⁸O_corr (black), and China composite δ¹⁸O (blue). Scatter plots of China composite δ¹⁸O vs. b HH-1 δ¹⁸O and c HH-1 δ¹⁸O_corr. The Pearson correlation coefficient (r) and Spearman’s rank correlation coefficient (ρ) are displayed in the bottom right corner of panels (b) and (c). For the correlation coefficient values, the China composite record was interpolated to the time steps of the HH-1 record, and Holocene values (gray circles) were excluded. The p-values for all coefficients are < 0.01. See Supplementary Table S2 for r values using δ¹⁸O_corr derived with different α_{calcite-water}.

Fig. 5. HH-1 compared to iTRACE simulations.

a Time series of HH-1 δ¹⁸O (orange), HH-1 δ¹⁸O_corr (black), iTRACE JJA δ¹⁸O_c (purple), iTRACE SON δ¹⁸O_c (light blue), and iTRACE annual δ¹⁸O_c (red). iTRACE δ¹⁸O_c values (VPDB) calculated from mean weighted iTRACE δ¹⁸O_p (VSMOW) and temperature output using α_{calcite-water} from ref. . For visual clarity, iTRACE curves are smoothed with a 100-year running mean. Scatter plots of iTRACE δ¹⁸O_p vs (b–d) HH-1 δ¹⁸O (orange) and (e–g) HH-1 δ¹⁸O_corr (black). iTRACE output was smoothed with a 50-year running mean. The Pearson correlation coefficient (r) and Spearman’s rank correlation coefficient (ρ) are displayed in the bottom right corner of each panel. The p-values for all coefficients are < 0.01. See Supplementary Table S2 for r values using δ¹⁸O_corr derived with different α_{calcite-water}.