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

• Published in: Chemical geology Vol. 168; no. 1; pp. 37 - 48
• Main Authors: Whipkey, C.E., Capo, R.C., Chadwick, O.A., Stewart, B.W.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.07.2000
• Subjects: Aerosol; Carbonate; Marine; Soil; Sr isotopes; Weathering; Soil; Weathering; Carbonate; Marine; Aerosol; Sr isotopes
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/S0009-2541(00)00187-X

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% MgCO 3). 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.