Orbital frequencies in radiolarian assemblages of the central Indian Ocean: implications on the Indian summer monsoon

Abstract

Radiolarian distribution in surface sediments and its relationships with overlying surface oceanography provided impetus for down-core faunal variation related to changes in monsoon intensity due to the Earth's orbital eccentricity in the geologic past. In present study, a high resolution (delta t=4.75 ka) radiolarian distribution for the last 485 ka in a core (AAS 2/3) from the central Indian Ocean Basin revealed orbital and sub-orbital cycles. Relationships between temporal distribution of: (1) high-salinity Pyloniids and Didymocyrtis; (2) low-salinity Spongodiscids and Spongaster; (3) transitional Euchitoniids; and (4) southern Anthocyrtidium assemblages, and the orbital forcing (ETP, i.e. the normalized sum of the Earth's eccentricity, tilt and precession), and the May-July insolation were analyzed. Spectral analyses revealed significant cycles of Pyloniids (400-, 126-, 95-, 54-, 41-, 31-, 23-, 19-, 17-, and 15-ka), Spongaster (95-, 41-, 29-, 23-, 19-, 17-, and 15-ka), Euchitoniids (126-, 95-, 51-, 31-, 21-, 18-, and 17-ka), and Anthocyrtidium (400-, 69-, 41-, 31-, 23-, 19-, and 15-ka). Cross-spectral analysis between Pyloniids and ETP suggested coherent Pyloniid cycles lagging ETP by 9 ka at 100-ka eccentricity, while both were in-phase (less than 2 ka) at 41-ka tilt and 23-ka precession cycles. Coherent Spongaster cycles led ETP by 38 ka at 100-ka eccentricity, 15 ka at 41-ka tilt, 5 ka at 23-ka precession cycles. Coherent Anthocyrtidium cycles led ETP by 14 ka at 100-ka eccentricity, 10 ka at 31-ka tilt, and were in-phase at 41-ka tilt as well as 23- and 19-ka precession cycles. Similarly, a radiolarian monsoon index defined as the normalized sum of Pyloniids, Didymocyrtis, Spongodiscids, Spongaster, Euchitoniids, and Anthocyrtidium (PDSSEA) led ETP at 54- and 23-ka cycles, while both were nearly in-phase at 41-, 31- and 15-ka cycles. Coherent PDSSEA cycles led insolation during May-July at 8 degrees S (core-site) at 23-ka precession, lagged 100-, 54- and 29-ka cycles and were in-phase at 41-ka tilt and 15-ka sub-orbital cycles. The results suggest that radiolarian assemblages exhibit proxy-monsoon cycles due to the Earth's orbital eccentricity at 400-, 126- and 95-ka, tilt at 54-, 41- and 31-ka, next to precession at 23-, 19- and 17-ka as well as insolation forcings at the core-site during the Late Quaternary.