LINKED HYDROCLIMATE AND PRIMARY PRODUCTIVITY OF OFFSHORE AND NEARSHORE LAKE TANGANYIKA DURING THE MIDDLE TO LATE HOLOCENE

The recent decrease in productivity of Lake Tanganyika exceeds the range of historical records. High-resolution paleoclimate and paleoecology archives could illuminate the link between hydroclimate and primary production in pre-anthropogenic and anthropogenic climate states. Here, we present analyses of TEX86 (a proxy for lake temperature), scanning XRF of Fe/Mn, Si/Ti (indicators for lake mixing and nutrient availability), K/Zr (an indicator for terrigenous input) and diatom records and hyperspectral imaging (HSI)-inferred chlorophyll (indicators for primary production) from two Holocene Lake Tanganyika sediment cores. Core 6A is a deep-water (middle to late Holocene) core and core 53A is a shallow-water (late Holocene) core. Core 53A was obtained from a site close to a small seasonal river. Also, we present a TEX86 record of core 1A which is from nearshore but farther away from the same river at 53A.

For 6A, low values of Fe/Mn indicate lake bottom ventilation associated with wind-driven water column mixing and upwelling. High values of Si/Ti indicate elevated nutrient availability. High values of HSI-inferred chlorophyll indicate elevated levels of total primary production. These changes are concurrent with indications from diatoms of increased productivity. Together, these records suggest that climate oscillations and lake productivity are tightly coupled.

For 53A, low values of Fe/Mn indicate lake bottom ventilation associated with wind driven upwelling between 340 and 560 AD, and river input between 560 and 1990 AD. Low values of K/Zr indicate increased terrigenous input. Increased terrigenous input is associated with indications of increased lake temperature, lake bottom ventilation, nutrient availability, and primary production, all of which decrease at the top of the core.

TEX86 for core 1A shows a general increase in lake temperature between 1850 and 2020 AD, which is not observed at site 53A.

Our results suggest that nearshore productivity (close to rivers) may be controlled by both internal and external nutrient loading, in ways that contrast with the open water portion of the lake (where thermal contrasts in the water column are the dominant control on vertical mixing and productivity). Thus, nearshore areas of the lake may respond differently to ongoing climate warming than the offshore region.