Mudbank off Alleppey, India: A bane for foraminifera but not so for carbon burial

Rupal Dubey; Rajeev Saraswat; Rajiv Nigam

doi:10.1016/j.scitotenv.2018.03.365

Mudbank off Alleppey, India: A bane for foraminifera but not so for carbon burial

Sci Total Environ. 2018 Sep 1:634:459-470. doi: 10.1016/j.scitotenv.2018.03.365. Epub 2018 Apr 6.

Authors

Rupal Dubey¹, Rajeev Saraswat², Rajiv Nigam²

Affiliations

¹ Micropaleontology Laboratory, Geological Oceanography Division, National Institute of Oceanography, Goa, India. Electronic address: rdubey@nio.org.
² Micropaleontology Laboratory, Geological Oceanography Division, National Institute of Oceanography, Goa, India.

PMID: 29631136
DOI: 10.1016/j.scitotenv.2018.03.365

Abstract

Calm conditions and extensive fishing, during monsoon season in the mudbank off Alleppey (Kerala), India creates a unique environment, associated with high suspended particulate matter. The effect of processes associated with mudbank formation, on benthic foraminifera, however, has not been documented. We have studied, seasonal foraminiferal distribution, to understand foraminiferal response to physico-chemical changes associated with the mudbank formation. Additionally, seasonal changes in total carbon, calcium carbonate (CaCO₃), organic carbon (C_org) and C_org/nitrogen (C_org/N) were also measured to understand the effect of mudbank formation on carbon burial. We report a low foraminiferal abundance in the mudbank. Benthic foraminiferal diversity is also low in the mudbank, during both pre-monsoon and monsoon season, clearly suggesting a stressed environment. Agglutinated foraminifera dominate the living benthic foraminiferal population in the mudbank, suggesting that the area is carbonate undersaturated and under fresh-water influence. Ammobaculites dilatatus and Ammobaculites exiguus are the dominant agglutinated species abundant in the mudbank and thus can be used to reconstruct past changes in the mudbank. The CaCO₃ is consistently low during all seasons, at one of the core mudbank stations. The %C_org is, however, higher in the core mudbank as well as the northern peripheral region. The C_org/N is consistently uniform at all the stations indicating a similar source of organic matter in all the seasons. The higher %C_org and constant C_org/N suggest, that food availability and its source is not a major factor affecting benthic foraminifera in the mudbank. Instead, increased turbidity and low bottom water salinity are the main cause of seasonally stressed environment in the mudbank. Additionally, C_org degradation coupled with fresh water influx induced drop in bottom water pH is responsible for low foraminiferal population in mudbank region, in all the seasons. The reduced calcareous benthic foraminiferal abundance, however, does not affect the carbon burial in the mudbank, due to higher %C_org.

Keywords: Alleppey; Benthic foraminifera; C(org); Low salinity; Mudbank.