WHEREFORE ART THOU, LACUSTRINE SILICA? MODELING AUTHIGENIC, BIOGENIC, AND DETRITAL SILICA SIGNALS IN QUATERNARY EAST AFRICAN LAKES

In overfilled lakes with circum-neutral pH, sedimentary silica flux is dominated by biogenic or clastic input. Ratios such as Si/Ti, therefore, generally indicate the approximate ratio of biogenic to clastic input. In contrast, end-member underfilled lakes (most of which are alkaline due to geologic setting) often have significant authigenic silicate precipitation (e.g. clays or Magadi-type chert). High pH in such lakes ensures significant or complete dissolution of biogenic silica. Si/Ti in this case reflects the ratio of authigenic to clastic input.

For both end-members, Si/Ti changes are due to the fluxes of authigenic or biogenic silica versus clastics (Ti). Fluxes increase proportionally at the expense of the others, so Si and Ti negatively covary downsection. Major limnological changes can temporarily destroy the relationship. For example, when a closed basin lake finds an outlet and transitions toward freshwater, Si and Ti covariance initially breaks down as all three sources may be significant. As a new steady state emerges, either authigenic or biogenic flux ceases to be significant, and Si/Ti covariance resumes.

To test this idea, a 5-pt. running average of a 5-pt. running correlation coefficient between whole-rock SiO₂ and TiO₂ was examined from time-series geochemical XRF data (Berry, 2012) from Olduvai Gorge, Tanzania, which hosted an underfilled Plio-Pleistocene paleolake (~1.92-1.76 Ma). SiO₂:TiO₂ R² values were mostly >0.50, but in places dropped to <0.01. These intervals correlate with terminations of peak aridity based on Al₂O₃/MgO, a proxy for saline authigenic clays. During these events, therefore the otherwise strong relationship between SiO₂ and TiO₂ broke down.

Data analysis is now underway from other basins in the region that are much fresher water systems. In Late Pleistocene Lake Malawi (Brown, 2011) it appears that SiO₂ and TiO₂ are also tightly coupled (R²>0.50), except for a number of regularly spaced events. Work is now underway to identify the nature of the intervals when the relationship broke down.

Elemental ratios such as Si/Ti are well known as important proxies of various sedimentological processes. The statistical approach used here adds value to geochemical datasets, and provides another important perspective on understanding paleoenvironmental change in lake basins.