Shell size variation of pteropod Heliconoides inflatus as a proxy for carbonate saturation

Abstract

The current study is an effort to understand the relationship between the average shell size of the pteropod species, Heliconoides inflatus as a metric for shell calcification using several cores collected from the northern Indian Ocean (NIO). The shell size variability and dissolution proxies provide in-life or post-depositional H. inflatus shell dissolution records. They can be utilized as biological indicators of aragonite saturation of the seawater column. The results suggest that the shell calcification rate of H. inflatus is controlled by atmospheric CO2 concentration and water column chemistry if the cores are above the aragonite lysocline (Aly). The variability in carbonate saturation can also be delineated using dissolution indicators such as the Limacina Dissolution Index (LDX) and fragmentation ratio (FR), given that the shells hadn’t experienced any breakage during post-depositional and sample processing. The results suggest that shell size and optimum growth conditions are directly related since larger shell sizes coincide with the least dissolved pteropod shells during lower atmospheric CO2 concentration periods. The surface water supersaturated with carbonate ions resulted in higher calcification rates, producing larger, thicker, and better-preserved shells. The calcification proxy complements the dissolution proxies and reveals that the most intense aragonite dissolution occurred during the Holocene and interstadials/interglacials. The severe aragonite dissolution during interglacial could be due to the CO32− undersaturated bottom waters. However, the shells have undergone preferential dissolution to smaller shells below the aragonite compensation depth (ACD), which is suggested by the highly dissolved shells and larger shells in the deep core samples.