Bathymetry and sediments on the carbonate platform off western India: Significance of Halimeda bioherms in carbonate sedimentation

Abstract

Bathymetry across the carbonate platform off western India indicated small-size pinnacles and their lateral coalescence into 2-6-m high mounds landward, and linear elongated carbonate ridges and troughs, mounds and banks up to a height of 20-m seaward of the platform. Seismic data indicated that these mounds were transparent with no rigid internal structure and can be defined as bioherms. The sediments were abundantly aragonite faecal pellets, Halimeda grains and ooids and their radiocarbon ages ranged from 11 to 7.5 ka BP. It appears that the growth of Halimeda bioherms on the platform was facilitated by intense upwelling during the early Holocene. The terrigenous sediments brought by rivers were deposited in the inner shelf and have not affected the growth of bioherms. It is estimated that the platform comprises at least 1.85 Gt of mass CaCO 3 accumulated during the early Holocene and comparable to those on the Great Barrier Reef. Halimeda bioherms produce abundant carbonate sediments and their growth period represents a geological carbonate sink and release of high CO 2 to the atmosphere. Detailed shallow seismic studies and sediment cores are needed to quantify the exact mass content of CaCO 3 and model climate change during the early Holocene.

Late Quaternary sediments on the carbonate platform off western India: Analogues of ancient platform carbonates

“…It is possible that a change in climatic conditions may have resulted in excess availability of nutrients (Hallock and Schlager 1986) that produces blooms of heterotrophic organisms, which in turn reduce the growth of reefs and carbonate secreting benthos. It is therefore suggested that the shift in upwelling centers and sea level changes after 7.5 ka BP may have led to their demise (Rao et al 2018). These authors also estimated the mass calcium carbonate content on the platform as 1.8 Gt, comparable with that on the Great Barrier Reef during the Holocene.…”

“…Detailed bathymetric studies on the platform revealed linear and laterally coalesced pinnacles in the form of mounds, well-developed large lens-shaped mounds (figure 4A and B), ridges and banks (Nair 1975;Vora and Almeida 1990;Rao et al 2018). These features were 2-6 m high landward and up to a height of 20 m seaward to the platform.…”

“…The limited fossil record of Halimeda mounds and several Holocene occurrences hint at specific climatic and environmental conditions responsible for their favored growth and preservation. Rao et al (1994Rao et al ( , 2018 investigated carbonate sand buildups on the carbonate platform and identified Halimeda bioherms formed during the Late Pleistocene-Holocene transgression and specified the environmental conditions responsible for their growth and demise.…”

“…Thin sections of the sands exhibit their internal structures that indicate most of the sands were Halimeda grains (figure 5A), followed by faecal pellets (figure 5B) and a few ooids and peloids (Rao et al 1994). On the basis of bathymetry and sediments, Rao et al (1994Rao et al ( , 2018 interpreted that the Halimeda bioherms on the platform produced abundant carbonate sands and mud and, the mud was subsequently fixed by Crustaceans as faecal pellets and peloids. The age of the carbonate sands mostly ranges between 11 and 7.5 ka BP and is presumed as the growth period of Halimeda bioherms.…”

“…The age of the carbonate sands mostly ranges between 11 and 7.5 ka BP and is presumed as the growth period of Halimeda bioherms. Rao et al (2018) suggested that the nutrients brought by upwelling currents are responsible for their luxuriant growth. The growth of bioherms stopped after 7.5 ka BP on the platform and subsequently became drowned, like many other drowned carbonate platforms common in the geological record.…”