Nature of organic matter preservation in coastal sediments: Insights from humid to arid climatic regimes of India.

Coastal sediments act as a huge reservoir of sedimentary organic carbon (SOC). Yet, the processes governing the long-term burial and preservation of this type of coastal organic carbon are complex and depend on the interplay between several factors, such as sedimentation rates, regional climatic conditions, post-depositional biological activity, and degradation. All these factors can significantly vary spatio-temporally within local micro-environmental conditions. The present study aims to understand the preservation of organic matter from two distinct climate zones of tropical India, the humid eastern lower Gangetic floodplain containing mangroves (average annual precipitation ranging ~1200–1600 mm) and the dry western Kachchh basin, which is essentially a salt flat (average annual precipitation ranging ~ 200–400 mm). At both places, sedimentation rates, total organic carbon (TOC), separated labile and refractory fractions of SOC (through chemical oxidation method), and their stable carbon isotopic (δ¹³C) compositions have been compared. The available data, along with the results from the present study, show that the sedimentation rate patterns through the Holocene are comparable at both study sites, showing higher rates (~0.4–0.6 cm/yr) up till Mid-Holocene which decreases to <0.05 cm/yr during the Late Holocene. The TOC ranges from 0.1 to 1.0% in Kachchh (average ~0.4%) and 0.2 to 2.1% in the lower Ganges floodplain (average ~0.6%). However, when the bulk and refractory fractions of SOC are compared, they reveal distinct patterns. The Gangetic floodplain exhibits a difference in the bulk and refractory δ13C values as it contains more labile and partially decomposed organic matter. In contrast, the Kachchh sediment shows consistent δ13C values in both the bulk and refractory fractions, indicating negligible presence of labile or decomposed organic matter. These findings suggest that in arid climates, the SOC is predominantly deposited in oxidized conditions, thus comprising mainly the refractory fraction of organic matter. At the same time, in humid environments, the SOC includes a mixture of labile and partially decomposed organic matter. This comparative study provides an example of how climatic variability plays a critical role in shaping SOC characteristics in otherwise similar depositional settings and emphasizes the importance of studying both labile and refractory fractions of SOC for reconstructing past climates.