TIMING AND FORENSICS OF AN IMMENSE AND RAPID SEDIMENTARY PROGRADATION OF THE NE LANA'I (HAWAIIAN ISLANDS) COASTLINE

Understanding how human activities affect sedimentary pathways is critical to understanding the future of landscape evolution in the anthropocene. To examine this, we present ²¹⁰Pb and ¹³⁷Cs chronology from a rapidly prograding sedimentary feature on the island of Lana’i, Hawaiian Islands, to determine potential anthropogenic causes of increased sedimentation on the island. Potential anthropogenic agents of increased sedimentation include the burning of forested and agricultural land by Hawai’i islanders during a war with Maui islanders (1778), clearing of 3,000 acres for a short-lived sugar plantation at the turn of the 20^th century, and subsequent uncontrolled grazing of the cleared lands by ungulates until the 1970’s. Anecdotal evidence of high sedimentation rates includes burial of three out of four stairs leading up to a raised foundation church built c. 1903, however no rigorous quantification of sedimentation rates has been performed. Several cores were taken in a transect perpendicular to the coast at 27, 258, and 559 m from the present shore. Except for the furthest inland core, these cores reveal coarse beach gravel, recognizable by carbonate shells and fragments, and siliciclastic/carbonate sand overlain sharply by a compacted reddish-brown clay. This contact occurs at an average depth of 0.398 m in the furthest shoreward core and at an average depth of 2.05 m in the three cores taken from the middle site. In the furthest inland core, the reddish-brown clay spanned the entire core to a depth of 4.78 m. To test for correlation between the increase in sediment and the potential anthropogenic agents, we are creating a depositional history using ²¹⁰Pb and ¹³⁷Cs, which has a half-life of 22.3 years. In addition, we are conducting carbon isotope analyses of sedimentary organic material, which can differentiate between sugarcane (C4) and native vegetation (C3). Initial results from our cross-contour transect will allow us to constrain the volume and timing of deposition, to ascertain the potential cause, and to establish a framework for analysis of cores taken from along a coastal transect and from offshore.