Holocene Paleoenvironmental Reconstruction From Mexico's Pacific Coast-A Paleotempestological Investigation

Abstract

This dissertation examines the paleoenvironments of four lagoons from Mexico’s Pacific coast, with the aim of retrieving sediment deposition from storm surge events to determine long-term tropical cyclone (TC) records. Lagoons Agua Dulce, Boquita, Mitla, and Nuxco are located along a 700 km stretch in states Jalisco and Guerrero. Roughly 70 meters of sediment were collected and subjected to multiple proxies, including loss-on ignition, a microfossil survey, and geochemical analysis. Nuxco’s dynamism is caused by intense and prolonged rainfall (largely from TCs), responsible for increasing water level, opening the tidal inlet, and draining the site (termed “blowouts”). High amounts of shell hash entering Nuxco’s nearshore from ~1280-~530 yrs BP suggests turbulent conditions, likely from recurrent blowouts caused by a wet climate and frequent TCs. A decrease in nearshore hash from ~530 yrs BP to present suggests a more stable environment from a drier climate and less TCs. Mitla and Boquita’s backbarrier environments were formed ~5200 yrs BP during a period of stabilizing sea level. The longest core extracted from Mitla reveals peat alternating with lagoonal clay layers, indicating limnic environments (inferred 4430-4220 yrs BP, 4080, 3950, 3680-3480, 3170-3080, 2990-2870, 1570-present) triggered by increases in precipitation. Limnic phases show good temporal correlation to wet periods determined from coastal and upland paleorecords, and long term El Niño records, a main cause of wet conditions, along with frequent and wetter eastern north Pacific TCs. Mitla’s cores lack overwash evidence from TCs and tsunamis, posing questions regarding the documented attribution of clastic deposition to extreme events here. Similar to Mitla, Boquita’s sediments indicate changes in water level. Namely, a desiccated blue clay section adjacent to gray clay with low water and organic contents, and high Cl concentration, suggests low water level and dry conditions from ~3150-1030 yrs BP. While Laguna Agua Dulce is the only site susceptible to overwash, individual events cannot be deciphered due to its sandy sediments and typical resuspension from strong currents. Findings from this dissertation shed light on the insensitivity of these lagoons to overwash processes from extreme events, while improving regional paleoclimatological and paleoenvironmental records and understanding.