Benthic foraminiferal assemblage response to the Early-Middle Pleistocene sea-level variations in the south-eastern Indian Ocean

One of the most studied and debated time intervals in our planet‘s recent climate history is the Early-Middle-Pleistocene Transition (EMPT). It represents a significant reorganization in the global climate system between 1.2-0.7 Ma, without any considerable variation in the orbital parameters controlling Earth’s insolation. This climate rearrangement is exemplified by a strengthening of ice ages and a switch in the periodicity of the glacial/interglacial changes from 41 kyr to a quasi-100 kyr cyclicity. The causes of the onset of this global climatic transition remain unclear, and its impact on equatorial to mid-latitude shelf areas is, to date, scarcely investigated.

IODP Site U1460 (27°22.4949′S, 112°55.4296′E; 214.5 mbsl) is located in the south-eastern Indian Ocean, on the uppermost slope of the Carnarvon Ramp. The study area is of particular interest, as it allowed the recovery of an expanded EMPT section and appears to have never been exposed to sub-aerial conditions during the sea-level lowstands during the glacial phases. Therefore, it is well-suited to assess the sensitivity of the western Australian carbonatic platform at high resolution to the EMPT sea level fluctuations.

The studied sedimentary sequence represents a time interval between 639.91 and 1092.34 ka, from marine isotope stage (MIS) 16 to MIS 32. For this period, we reconstructed the plankton/benthos (P/B) ratio to qualitatively establish the eustatic fluctuations in the region, as highstand and lowstand stages correspond to higher and lower values of the P/B ratio, respectively.

The P/B ratio has been coupled with a benthic foraminiferal assemblage study to ascertain the ecological variations in the area and their link to the glacial-interglacial-induced sea level oscillations. Benthic foraminiferal assemblages revealed a polyspecific and highly diversified assemblage. Specifically, within the studied interval, we recorded more than 318 species, most of which are represented by <1% abundance. Preliminary data allowed to distinguish a first part of the record (MIS 27-23) dominated by Cibicidoides spp., Heterolepa spp., Trifarina bradyi, and nodosarids, from the most recent interval (MIS 22-16), which recorded abundant agglutinated tests (e.g., Gaudryina spp., Textularia spp., Spirotextularia spp.), Cibicidoides spp., Heterolepa spp., Siphogenerina spp., uvigerinids and bolivinids. Other common taxa are Lenticulina spp., Nuttallides umbonifer, cassidulinids and lagenids.

Benthic assemblage analysis revealed that diversity remains high throughout the record (average Shannon H´ = 4.24) but tends to decrease during interglacial intervals (average Shannon H´ = 4.001). Such a diversity decrease supports the persistence of warmer, more tropical conditions during interglacials at the study Site. Agglutinated tests’ abundance increases towards the most recent part of the record, but no significant changes in their abundance are noticeable at the glacial-interglacial turns. Conversely, the increase of dominance during the interglacial phases (MIS 17, MIS 19 and MIS 21) in the youngest part of the record seems to be linked to an increase in the uvigerinids abundance and a concomitant decrease in Cibicides/Cibicidoides abundance. The oldest part of the record (between 1092.34-755.59 ka) registered an important decline in the number of uvigerinids and a contemporary increase in the abundance of Trifarina bradyi and cassidulinids.