Influence of tectonics and atmospheric deposition on nutrient sources to tropical forests of the Osa Peninsula, Costa Rica

The warm, wet climatic conditions that determine tropical forest distribution also rapidly leach nutrients out of tropical soils. Over geologic time scales, nutrient stocks originally supplied by mineral weathering become depleted and the dilute inputs of nutrients in atmospheric deposition become increasingly important to sustaining ecosystem productivity. I examined this transition from rock- to atmospherically-derived nutrients in rainforests of the Osa Peninsula, Costa Rica, a landscape with an extremely unusual tectonic history. Prior to 32 kyr ago, sections of the Osa Peninsula were stable or slowly subsiding into the ocean, and a thick and highly weathered mantle of regolith formed from basalt bedrock. Over the last 32 kyr those same sections of the peninsula were rapidly uplifted. Steepened stream profiles rapidly eroded the landscape and remnants of the thick, weathered mantle exist now as highly weathered soils on ridges and slopes.; Using new and previously tested techniques I estimated the proportion of elements in Osa Peninsula forests that entered the system via rock weathering or atmospheric deposition. Measurements of strontium isotopes revealed that ∼90% of this element in forests is still rock-derived despite the advanced weathering state of the soil and measurable inputs from deposition. This condition is the result of the removal of overlying soils which had likely become dominated by atmospheric strontium, and a short surface exposure time for the modern soils. Sulfur isotopic measurements suggested that atmospheric deposition accounts for nearly all of this nutrient on the same soils. This contrasting result appears to derive from the high efficiency with which past atmospheric deposition of sulfur was retained throughout deep weathering profiles. Finally, I estimated the different source contributions of the nutrient cations to the same forests. This work suggests that 85+/-20% of calcium, 90+/-30% of magnesium, and 70+/-15% of potassium being cycled through vegetation are still derived from rock weathering.; These results show that the transition to dependence on atmospheric nutrients is subject to the characteristics of the element in question. The results also describe the effects of an unusual landscape history on the modern biogeochemical cycles of a particularly rich region of tropical rainforest.