Palaeosalinity and palaeoclimatic geochemical proxies (elements Ti, Mg, Al) vary with Milankovitch cyclicity (1.3 to 2.0 Ma), OGCP cores, Palaeolake Olduvai, Tanzania

摘要

The analysis of geochemical palaeoclimate and palaeosalinity proxy elements Ti, Mg, and Al, derived from X-ray fluorescence (XRF) scans of Olduvai Beds I and II from Olduvai Gorge Coring Project (OGCP) borehole Cores 2A and 3A, provides a record of cyclic variation between similar to 1.3 Ma and similar to 2.0 Ma. The boreholes were drilled into the depocentre of the Olduvai Basin between Fifth and FLK Faults, where Palaeolake Olduvai was most persistent and deepest. During most of Bed I the lake was particularly deep and probably meromictic, preserving high TOC contents and commonly preserving fine lamination due to lack of bioturbation. Accretion rates were also high during Bed I, when rates of basinal subsidence were maximal due to crustal stretching, associated with basaltic volcanism, towards the end of bimodal Ngorongoro volcanism. Basaltic magma effusive activity is manifested as tuffs, scoriaceous layers and the Bed I Basalt complex lava flows. A magnesium anomaly is recorded in the claystone geochemistry at this time and deposition of dolomite and limestone beds are restricted to this synvolcanic phase of basin history. During Bed I deposition, accretion rates (0.23 mm/yr) were high enough to permit recognition of cycles with an average periodicity of 22.3 kyr corresponding to the Earth's precession. Only the high values during the Mg anomaly are adequate for the application of the palaeosalinity proxy element ratio Mg/Al. But Ti counts provide a cyclic record in both Bed I and Bed II. During Bed II deposition, the accretion rate was much slower (0.058 mm/yr) and cyclicity averaged 40.4 kyr, corresponding to Earth's orbital obliquity. The Bed II interval corresponds to MIS Stages 40 to 64. Coincidence of precessional and obliquity minima at 1.8 Ma explains the superdrought that affected the basin at the time of emplacement of Tuff IF, when the lake was dried out. The aridity of the sequence containing Tuff IA is also associated with a precessional minimum. The cyclic record suggests that three Bed I Basalt flows were extruded at similar to 1.94 Ma during a time span lasting between 6 kyr and 15 kyr.