Radiolarian abundance - A monsoon proxy responding to the Earth`s orbital forcing: Inferences on the mid-Brunhes climate shift

Abstract

Radiolarian fluxes from the sediment traps moored in the southern Bay of Bengal suggested that radiolarians were invariably higher at times of higher sea surface temperature (SST greater than or equal to 28 degrees C) and moderately lower salinity (approx. 33.5 psu) during the summers of 1991-93. It implied that higher SSTs due to summer heat and lower salinity due to rains might have affected the radiolarian fluxes during the monsoon. Hence, the down core radiolarian-yield in the unit weight as the percentage abundance may shed light on monsoon-sensitive radiolarian production through the geological past. It is tested in a well-dated radiolarian ooze-bearing sediment core collected from the central Indian Ocean in the southwest of the trap site. Radiolarian tests/g dry bulk sediment (Rads/g) yield from the sediment samples at every 4750 years (4.75 ka) time interval (delta t) produced a time series for the last 485000 years (485-ka) during the late Quaternary. Rads/g was higher during the upper last 200-ka record than the lower section of the core implying higher radiolarian production in the upper part than in the lower part of the core. Blackman-Tukey and maximum entropy spectral analyses of the Rads/g time series revealed multiple cycles at approx. 400-, 126-, 95-ka due to the Earth’s orbital eccentricity, approx. 54-, 41-, and 31-ka due to axial tilt, 23-, 19-, and 17-ka due to changes in the precession of equinoxes, besides the 15-, 13- and 11-ka sub Milankovitch cycles. The spectral results are further verified by multi-taper method using 90% reshaped spectrum, adaptive narrowband and high-resolution harmonic signals with robust noise and the statistical significance test (F-test), and the >90-99% significant cyclic components are reconstructed to visualize the variation in their cyclic amplitude through time. The reconstructed cyclic components revealed a conspicuous change in the trend of the amplitudes of approx. 95-, 54-, 41-, 17-, 15-, and 13-ka cycles suggesting an effect of the mid-Brunhes epoch climate shift at around approx. 300-350-ka before present. The results substantiates the existence of multiple cycles at the eccentricity (approx. 400-, 126-, and 95-ka), tilt (approx. 54-, 41-, and 31-ka) and precession (23-, 19-, and 17-ka) and sub-orbital bands (approx. 15-, and 13-ka) as proposed by Berger (1977) in the astronomical data. These results strengthens the growing belief in multiple orbital cycles reported from pollens (Mommersteeg et al, 1995) and the radiolarian assemblages (Gupta, 2003), suggesting the cyclic changes in the summer-monsoon productivity during the Late Quaternary in the Central Indian Ocean sediments