Coherent nature of speleothems petrographic and isotopic proxies for climate change: A case study from northwestern Himalaya

Abstract

Speleothems are widely used terrestrial archives for the reconstruction of regional paleoclimatic conditions using mainly stable oxygen isotopes. However, the isotopic signals are quite vulnerable to post‐depositional recrystallization, and in the absence of detailed petrographic characterization, the validity of the isotopic dataset remains elusive. The present study is an attempt to bridge the gap between speleothem physical and geochemical proxies, their interrelationship and applicability, to reconstruct the regional climatic conditions in the northwestern Himalayas. We have identified different zones of depleted/enriched oxygen isotopes linked with the enhanced/decreased regional rainfall with strong/weak Indian summer monsoon. Prograding growth laminae and distinct columnar crystalline texture in calcite are associated with depleted oxygen isotopes. This suggests ample water availability and a higher drip rate in the cave, and further approves the wetter regional climate. Whereas, retractional growth laminae with dark micrite and microspar point towards slower drip rate and high evaporation associated with decreased rainfall. The presence of microspar in the studied stalagmite may indicate possible diagenetic alteration. However, oxygen isotopes data is unable to detect these small‐scale changes. Nevertheless, a coherency exists between isotopes and petrographic proxies for regional climate variability on a larger scale, and diagenesis in the studied sample possibly occurred during the early stage of precipitation. Therefore, isotopic studies coupled with detailed petrography can better ascribe paleoclimate conditions, and minimize the chances of erroneous interpretation because of diagenetic alterations of carbonate.