TAPHONOMICALLY- AND INFAUNALLY-ADJUSTED SALT MARSH FORAMINIFERAL RECORD OF LATE HOLOCENE EARTHQUAKE DISPLACEMENTS AND A TSUNAMI SAND, NEW ZEALAND

Abstract

Most paleoenvironmental assessments of fossil salt-marsh foraminiferal faunas are based on modern analogue samples from the surface 1–2 cm. Usually no account is taken of the faunal modifications that result from the 60–90% of the fauna that live at greater infaunal depths or from the patchy loss of agglutinated tests that mostly occurs in the oxic and taphonomically active zone (TAZ, upper 10–15 cm). Here we provide examples of the highly variable foraminiferal test distribution in New Zealand salt marsh cores and surface transects. Using these, we suggest two simple adjustments that could be made to quantitative estimates of paleoelevation derived from fossil salt marsh faunas based on modern surface analogues: 1) Determine a minimum dead specimen density (8 per cm3 in this study) below which all fossil faunas are considered to have suffered significant taphonomic loss (in the TAZ) and their paleoelevation estimates are deemed suspect and may be applied tentatively to core depths ~15+ cm above. 2) Move the core depths of all acceptable paleoelevation estimates upwards by the mean infaunal depth of the dominant species (2.5–8 cm in this study). We applied these adjustments to two late Holocene cores located 5 m apart in salt marsh in tidal Big Lagoon, Marlborough, New Zealand. Fifty percent (18) of the fossil faunas have unacceptably low specimen densities (<8 per cm3). A 15 cm thick fine pebbly sand bed contains a mix of calcareous tests derived from subtidally offshore and from intertidally within the adjacent sheltered lagoon, and agglutinated tests of salt marsh taxa inferred to be displaced and/or infaunally in-situ. These faunas provide strong evidence for sand deposition on top of the high salt marsh by the outgoing surge of a tsunami. The upper 7 and 13 cm of the sand lacks calcareous tests, which are inferred to have been dissolved by acidic pore waters following colonisation by salt marsh vegetation. Our taphonomically- and infaunally-adjusted paleoelevation estimates, generated by modern analogue technique using a training set of 1017 modern New Zealand faunas, provide near-identical elevational histories for both cores. Both core sequences (70 and 85 cm thick) were deposited within a 50 cm-wide elevational envelope in middle- and high-tidal salt marsh. Both record an inferred 0.5 m co-seismic subsidence event coincident with emplacement of the tsunami sand (~840 cal yrs BP) and a second smaller, ~0.3 m, presumably co-seismic, subsidence event coincident with a sharp peat-mud contact in one core (~500 cal yrs BP).