To infer long-term patterns of tropical cyclone activity, it is imperative to collect and interpret data beyond the historical observational period. Paleotempestology is a relatively young science that studies past tropical cyclone activity mainly by means of geological proxy techniques. Paleotempestological research has been successfully conducted at many locations along the U.S. Gulf and Atlantic coasts and the pan-Caribbean region, but proxy records are notably lacking from the Pacific coast of Mexico.
Doctoral student Thomas Bianchette under the supervision of Professor Kam-biu Liu will establish a centennial-to-millennial record of paleohurricane strikes from Jalisco and Guerrero on Mexico's Pacific coast using coastal lake-sediment proxies. The geomorphic setting of the study sites and the typical tracks and strengths of tropical cyclones in this region suggest that hurricane strikes will be identifiable in these sediments. In addition to using overwash sand layers as a storm surge proxy, this study will pioneer a new methodology using slopewash deposits and fossil pollen as supplementary proxies to detect indirect hurricane strikes resulting from coast-parallel storms that bring heavy precipitation but do not make landfall (i.e., limited storm surge). The methodology will be based on the hypothesis that increased storm activity during an active period on centennial timescales will result in a wetter climate, leading to a long-term vegetation response that can be detected in the pollen record. Recent storms that have caused a geological and/or biological imprint will be used as modern analogs to aid in interpreting older storm events in the sedimentary record. A geographic information system will be developed to visualize the relationship between historical storm tracks and climate conditions (especially El-Nino/Southern Oscillation), while remote sensing technologies will be used to determine the geomorphological and biophysical effects of recent hurricanes. We hypothesize that a longer return period will be found for tropical cyclones along this coast than for the U.S. Gulf and Atlantic coasts based on multi-millennial data, with northern areas (Jalisco) on Mexico's Pacific coast exhibiting a higher storm frequency than southern locations (Guerrero).
This research promises to produce the first proxy records of tropical cyclone activity along the Pacific coast of Mexico. Because unraveling global tropical cyclone activity depends upon inter-basin correlations, proxy records of long-term activity regimes for the Eastern North Pacific basin are vital. The new methodology of using slopewash deposits and fossil pollen as supplementary proxies can advance paleotempestological studies by permitting the use of inland sites and detecting paleo-storm events that brought excessive rain but did not cause significant overwash processes. Furthermore, this study will have great societal relevance as historical tropical cyclone strikes along Mexico's Pacific coast have caused tremendous damage to society and infrastructure, including significant public health problems. Mexico's instrumental record of these storms is very short (since 1949), making long-term risk assessment difficult. Establishing more reliable estimates of hurricane return periods based on multi-centennial or millennial proxy records can help inform decision makers, thereby improving planning and mitigation policy.
The main objective of this study was to reconstruct a multi-millennial sedimentary record of tropical cyclone (TC) activities for the Pacific coast of Mexico, and assessing the major climatic mechanisms controlling these activities in the context of global and regional climate changes during the Holocene. During October and December of 2009, over 70 meters of sediment was retrieved from four coastal lagoons (Mitla, Nuxco, Boquita, Agua Dulce) on Mexico’s Pacific coast covering a 700 km swath from Jalisco to Guerrero, representing the most comprehensive regional paleoenvironmental reconstruction for that coastal region to date. By using an array of geological, chemical, and biological proxies, this project has yielded a multi-millennial record of paleoenvironmental changes upon which reconstructions of long-term changes in TC activities can be based. Located 40 km WNW from the popular resort town Acapulco, Laguna Mitla is a backbarrier lake that offers a 5200 year sedimentary record. Discharge from a small incoming stream is mainly climate controlled. A sediment core extracted ~ 3 km from the stream contained iterations of riverine sediment (flooding signal) and peat, indicating vegetation development and low water level. These "flood" phases indicating increased discharge and a wetter climate occurred at ~4430-4220 years BP, 4080, 3950, 3680-3480, 3170-3080, 2990-2870, and 1570-present. Since hurricane precipitation accounts for a significant percentage of annual precipitation for western Mexico (exceeding 60% in some areas), we interpret wet climatic periods to be suggestive of increased eastern north Pacific hurricane frequencies, which could have been triggered by warm surface water and reduced vertical shear during El Niño. Frequent and intense floods throughout the last ~3500 years were caused by heightened El Niño intensity. Our study results suggest that the Acapulco area is currently under an active hurricane regime on a multi-millennial timescale. We found that during the past 5,000 years Mitla's active periods generally exhibit an antiphase temporal relationship with the paleohurricane record from Puerto Rico where high TC activity tends to correspond with decreased El Niño frequency. A paleoclimate record was also produced for Laguna Boquita, 60 km SW of Puerto Vallarta and 700 km WNW of Laguna Mitla. From ~3150 to 1030 yr BP Boquita contained iterations of crumbly blue and gray inorganic clay with high chlorine concentrations, suggesting lack of adjacent vegetation and low water level due to dry climate conditions. This dry period is in agreement with low water level from Mitla (2870-1570 yrs BP) and evidence of drought documented from regional upland and coastal paleoclimatic records. Increases in water level were suggested from 1030 yr BP to present, suggesting wetter conditions and an increase in hurricane frequency. This study has yielded the longest paleoclimatic records on Mexico's Pacific coast to date. Previous paleoenvironmental studies based on sedimentary records from Pacific coastal lagoons such as Laguna Mitla in western Mexico have attributed the occurrence of sand layers to prehistoric hurricanes or tsunamis. However, our multi-core examination of sites with similar sizes and morphologies do not support the presence of overwash sand layers in nearshore sediments. Instead, sand deposits were found only in cores extracted from the middle or backside of the lagoon, suggesting that they were formed by fluvial erosion of relict Pleistocene beach ridge segments currently exposed as many as 5 km landward of the present coastline. Our findings contradict previously published reconstructions regarding the occurrence of Holocene tsunamis and hurricanes in the study region, and have implications for risk assessment for coastal populations along the Pacific coast of Mexico.
