Monsoon-influenced terrestrial forcing shapes inter-reef carbonate production (Spermonde Archipelago, Indonesia)

Abstract. Monsoon-driven fluvial discharge exerts strong environmental control on water quality and carbonate sediment composition in tropical shelf systems, yet the capacity of photosymbiont-bearing calcifiers to sustain carbonate production under monsoon-amplified turbidity and nutrient loading across semi-enclosed platforms remains poorly understood. We here investigate inter-reef facies patterns and benthic foraminiferal assemblages across the Spermonde Archipelago (southwest Sulawesi, Indonesia), a land-attached tropical carbonate shelf subject to seasonal monsoon runoff and intensifying coastal development, using geochemical, sedimentological, and foraminiferal analyses of 51 inter-reef seafloor samples. Nearshore sediments are enriched in mud and organic matter, and dominated by heterozoan and stress-tolerant assemblages, yielding low values for the FoRAM Index (FI; an index that increases with the relative abundance of photosymbiont-bearing foraminifera), indicating conditions unfavourable for photosymbiont-bearing carbonate producers. Statistical separation of nearshore facies from all mid- and outer-shelf assemblages supports a threshold rather than gradual cross-shelf transition. This threshold is spatially asymmetrical, extending further offshore in the northern sector, likely reflecting enhanced retention of fluvially derived material within the more enclosed northern shelf geometry, with the southward Makassar Strait circulation limiting offshore dispersal. Mid- and outer-shelf deposits (~80% of the investigated area) are characterised by foramol facies dominated by benthic foraminifers, bivalves and gastropods, with Amphistegina and Operculina as the dominant symbiont-bearing foraminifera; FI values increase progressively offshore, reflecting improving photic conditions as terrestrially driven seasonal turbidity decreases with distance from the coast. Strong spatial correspondence between foraminiferal assemblages and independently derived sedimentary facies, including convergence of the foraminiferal community transition zone with foramol mud-supported carbonate sediment facies, validates the FI as an ecological proxy beyond shallow reef settings and demonstrates its utility in differentiating carbonate production modes in mesotrophic equatorial systems. These findings support a carbonate production continuum in which photozoan and heterozoan components are organised non-linearly along environmental gradients, with threshold responses emerging from the interaction of platform geometry and catchment-scale terrestrial forcing under seasonal monsoon modulation. We therefore interpret the Spermonde shelf as spatially structured by shifting autotrophic-heterotrophic balances, rather than discrete facies categories.