WEATHERING, MARINE, AND VOLCANIC EMISSION CONTRIBUTIONS OF SULFUR TO HAWAIIAN ECOSYSTEMS AND SOILS

Nutrient inputs exert fundamental controls over terrestrial ecosystems by influencing nutrient availability. It is sometimes assumed that weathering supplies the majority of rock-derived nutrients to ecosystems, but numerous studies also show the importance of atmospheric deposition. Sulfur is a nutrient whose deficiency can limit ecosystem productivity in certain situations, but its overabundance is linked to ecosystem acidification. We examined sources and abundance of ecosystem sulfur at sites varying in climate, soil age, and proximity to volcanic emissions in the Hawaiian Islands. Sulfur isotope ratios (d³⁴S VCDT) were used to trace sulfur sources. Weathering of fine-grained to glassy basaltic parent-material (-0.8‰) is one potential sulfur source. Atmospheric deposition sources could include emissions of volcanic SO₂ from Kilauea Volcano (+0.8‰), sea salt (+21.1‰), and marine non-seasalt sulfate (+15.6 ± 3.1‰). Sulfur isotope ratios in soils along the soil age gradient differed substantially. Young soils (0.3 ka) on the island of Hawaii had d³⁴S values of +1.0 ± 1.1 ‰, similar to basalt parent-material and volcanic SO₂. The oldest soils (4,100 ka) on the island of Kauai had d³⁴S values of +18.7 ± 0.5‰, similar to marine sulfur. Intermediate age soils had intermediate values. Soils at the dry (MAP 180 mm) end of a climate gradient had d³⁴S values of +9.3 ± 1.1‰ while values at the wettest site (MAP 2500 mm) were +13.7 ± 0.7‰. The differences observed at all sites could be driven by mixing of rock and atmospheric sulfur in soil, or mixing of marine and volcanic sources in atmospheric deposition. Analysis of soils developed on young lava flows on the island of Maui, farther from point sources of volcanic SO₂, is beginning to shed light on that question. The majority of sulfur in the climate and age gradient soils is immobilized in organic matter, and thus linked to its turnover. Despite differences in organic matter turnover times of 1-100 kyr between surface and depth, d³⁴S values vary little in individual profiles. This suggests relative stability in sulfur inputs on similar time scales. As a result, soil sulfur isotope ratios may be useful for tracing the spatial impact of vog (volcanic fog) in addition to tracing sources of ecosystem sulfur.