The importance of sea spray to the cation budget of a coastal Hawaiian soil: a strontium isotope approach

Soil nutrients such as Ca, Mg, and K are traditionally thought to be derived primarily from rock weathering. Here we show that sea spray is a significant source of nutrient elements to modern and buried soils developed on <30,000-year-old Pahala Ash deposits 50 m from the coast at South Point, Hawaii. The soil profiles evolved in a semi-arid climate and have always been above sea level and the water table. Rhizoliths (fossilized root traces) and horizontal laminated carbonate sheets found in buried soils are composed of high-Mg calcite (up to 14 mol% MgCO3). Differences in strontium isotopic composition between marine aerosols (87Sr/86Sr=0.7092) and tephra parent material (∼0.7035) allow quantification of cation sources to the labile soil reservoir and to pedogenic carbonate. Mixing equations indicate that 50–80% of labile soil Sr and approximately half of carbonate Sr was derived from marine sources. Using the Sr isotopic signatures and Sr/Ca ratios of seawater and tephra as end members, we determined that up to 2/3 of the Ca in the labile reservoir and up to 1/3 of Ca in the carbonates has a marine origin. Carbonate 87Sr/86Sr ratios are fairly constant with depth, but labile 87Sr/86Sr ratios indicate decreasing sea spray aerosol influence with depth. This trend could be due either to sequestering of aerosol-derived Sr in the upper part of the profile or to lower aerosol input in the past due to lower sea level. The unusual occurrence of high-Mg pedogenic calcite probably results from high labile Mg/Ca ratios during earlier stages of weathering, coupled with rapid calcite precipitation during soil pore water evaporation.