The influence of shallow-water methane emissions on foraminiferal assemblages: The case of Scoglio d’Affrica (Northern Tyrrhenian Sea, Mediterranean Sea)

Abstract

Microfaunal analyses were conducted near Scoglio d’Affrica in the Tuscan Archipelago (Northern Tyrrhenian Sea), to study the response of benthic foraminifera to methane (CH₄) venting activity that occurs in this shallow water environment. Our data show that sedimentary processes linked to the CH₄ emissions exert a strong influence on foraminiferal assemblages, resulting in a very patchy spatial distribution linked to complex abiotic and biotic interactions. Methane emissions and mud represent the two main stressor factors for the benthic foraminiferal assemblages, although at present it is not possible to determine which impact dominates.

Five different morphological settings, controlled by venting activity, were defined on and off the mud volcanoes (MVs). Each of these settings has distinct assemblages: 1) areas with strong emission activity at the top of the MVs, locally associated with gryphons and mudflows, where the environmental conditions are clearly prohibitive for foraminiferal life; 2) mud flows along the MV flanks, where overlapping mudflows likely limit foraminiferal colonization; 3) muddy sediments associated with weak emissions where the development of foraminiferal community is favored, although with differences in terms of density, diversity and compositional features linked to the timing of colonization by each species; 4) intermatte zones with scarce or absent emissions, characterized by typical shallow water taxa indicative of well-oxygenated and highly hydrodynamic conditions; and 5) Posidonia oceanica substrates, characterized by higher foraminiferal content on the leaves compared to the rhizomes and surrounding sediments; indeed, sediments and rhizomes were more impacted by emissions, whereas Posidonia leaves offer “refugia” and a more mitigated environment.

Although it is difficult to define a pattern of biota response and to identify seep-exclusive taxa, foraminifera can represent good environmental proxies for both monitoring the variability of recent venting activity and detecting stressed conditions occurring in the geological record. The seafloor around Scoglio d’Affrica represents a very promising study site for multidisciplinary marine research regarding venting activity, geochemistry of cold seep fluids and their effects on benthic organisms.