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 derive d primarily from rock weathering. Here we show that sea spray is a signific ant source of nutrient elements to modern and buried soils developed on < 3 0,000-year-old Pahala Ash deposits 50 m from the coast at South Point, Hawa ii. The soil profiles evolved in a semi-arid climate and have always been a bove sea level and the water table. Rhizoliths (fossilized root traces) and horizontal laminated carbonate sheets found in buried soils are composed o f high-Mg calcite (up to 14 mol% MgCO3). Differences in strontium isotopic composition between marine aerosols (Sr-87/Sr-86 = 0.7092) and tephra paren t material (similar to 0.7035) allow quantification of cation sources to th e labile soil reservoir and to pedogenic carbonate. Mixing equations indica te 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 ra tios 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 Sr-87/Sr-86 ratios are fairly constant with depth , but labile Sr-87/Sr-86 ratios indicate decreasing sea spray aerosol influ ence 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 th e past due to lower sea level. The unusual occurrence of high-Mg pedogenic calcite probably results from high labile Mg/Ca ratios during earlier stage s of weathering, coupled with rapid calcite precipitation during soil pore water evaporation. (C) 2000 Elsevier Science B.V. All rights reserved.