Sarah E. Metcalfe

Sta. Cruz Atizapán: a 22-ka lake level record and climatic implications for the late Holocene human occupation in the Upper Lerma Basin, Central Mexico

Abstract The Upper Lerma is a high altitude basin with three water bodies linked by the Lerma River. This basin has a long archaeological history, characterised by the establishment of settlements within the lacustrine ecosystem itself (man-made islands) during the late Classic to Epiclassic (AD 550–900), which were abandoned by the end of the Epiclassic. The Upper Lerma is an ideal site to study climatic and environmental conditions during the period of human occupation, as well as during the last full-glacial/interglacial cycle. Two sediment cores (STCRZ: 9.54 m and Almoloya del Rio: 5.12 m) were recovered from the highest lake in the system (Chignahuapan). Ten radiocarbon dates provide chronologies for these sequences in which the Tres Cruces Tephra (c. 8500 yr BP) and the Upper Toluca Pumice (c. 11 600 yr BP) serve as stratigraphic markers. Magnetic properties, loss on ignition, and diatom analyses were used to infer lake level fluctuations during the last c. 22 000 yr BP. The Late Pleistocene environment was characterised by a freshwater lake. High sediment input and variable lake levels are recorded during the Last Glacial Maximum (c. 19 000–16 000 yr BP), while slightly higher water levels and reduced sediment input are recorded during the Late Glacial (c. 16 000–11 000 yr BP). A short episode of shallow conditions is inferred by c. 12 400 yr BP. Holocene lake levels were generally shallower, and three episodes of very shallow, slightly alkaline waters are identified. The first dates to the early Holocene (c. 11 000–7000 yr BP). The second is centred at c. 4600/4500 yr BP. The third occurred between c. 2000 (?) and 800 yr BP (c. 200 BC–AD 1100, calibrated ages) with very shallow water after c. 1400 yr BP (AD 550, calibrated age). Lake level increased after c. 800 yr BP. These three shallow water events are also recorded at other sites in Central Mexico indicating regional climatic trends rather than local events. A deeper water phase occurred between 7000 and 6400/6200 yr BP. The last shallow water phase correlates with the Classic and Epiclassic periods (AD 200–900), and shallowest conditions occurred in the late Classic to Epiclassic (c. AD 550–900), when the construction of man-made islands reached a peak. An increase in lake level after c. 800 yr BP (AD 1100 calibrated age) may have led to the abandonment of this life strategy.

Climate variability over the last two millennia in the North American monsoon region, recorded in laminated lake sediments from Laguna de Juanacatlán, Mexico

High-resolution titanium (Ti) data obtained using an ITRAX XRF core scanner from a laminated sediment core from the Laguna de Juanacatlán, western central Mexico yield a unique high-resolution record of runoff (precipitation) change for the last 2000 years. In the absence of reliable, long-term meteorological records, comparison of the Ti data with information from the rich Spanish colonial period archives and the post-Independence period, confirms that Ti is a proxy for runoff. This interpretation is supported by comparison with other high-resolution archives from the surrounding region, primarily tree rings and other lake sediment sequences. The Juanacatlán Ti record is therefore a proxy for summer, monsoonal rainfall. The record provides new evidence from the Pacific margin of tropical North America of the occurrence of dry conditions through much of the Classic period (c. AD 300—900), and wetter conditions during the later Medieval period (c. AD 1200—1350). The period commonly known as the ‘Little Ice Age’ (LIA) shows considerable variability, with dry conditions in the early part (c. AD 1400—1600) and wetter conditions, punctuated by multiyear droughts through the eighteenth century. A notable feature of the record is the apparent decoupling of lacustrine sedimentation from the climate since the mid-twentieth century, possibly resulting from anthropogenic disturbance. Preliminary interpretations of the Ti record indicate that patterns are consistent with changes in monsoon strength associated with ENSO and solar forcing over the last two millennia.

Climate change at the monsoon/Westerly boundary in Northern Mexico

Northern Mexico lies close to the present day boundary between mid-latitude (Westerly) and tropical (monsoonal) sources of moisture. Studies from the adjacent southwest USA have shown major changes in lake levels and vegetation distributions over the late Quaternary which have been interpreted in terms of significant variations in the relative strengths and positions of these climate systems. Palaeoclimatic data from this area have, however, left a number of unresolved questions which can only be answered by extending work into northern Mexico, closer to the major source of summer (monsoonal) rain, the Gulf of Mexico. Studies of palaeolake sediments from a series of hydrologically closed lake basins across a range of altitudes (1280 to 2200 m a.s.l.) in northern Chihuahua are in progress using geochemical, mineral magnetic, diatom and plant microfossil analyses. Preliminary results are presented from the Alta Babícora and Encinillas basins. The sites provide records of lacustrine deposition between >11 000 and about 2500 yr BP. The diatom record from Babíora provides clear evidence for a deep water lake in this basin in the late glacial which persisted into the early Holocene. A dry episode coinciding with the timing of the Younger Dryas is recorded in Alta Babícora. Conditions wetter than present are indicated up to at least 7000 yr BP.

Holocene Climate Patterns in the Americas Inferred from Paleolimnological Records

Holocene paleolimnological records for the Americas that clearly reflect climate are limited in number, particularly for the Southern Hemisphere. Most lacustrine records are of variation in effective moisture (precipitation minus evaporation), except for the Arctic, where temperature is the dominant influence. The millennial-scale patterns of climate variation evident in paleolimnological records are consistent with changes in insolation, modulated in the early Holocene in North America by the influence of the Laurentide ice sheet. In the midlatitudes of both North and South America and in the Southern Hemisphere tropics, the mid-Holocene was drier than it is today, although the onset, magnitude, and duration of aridity vary considerably. In contrast, the Northern Hemisphere tropics were wetter than they are today at times during the mid-Holocene. The broader scale patterns of mid-Holocene moisture variation are consistent with a northerly migration of both the Intertropical Convergence Zone (ITCZ) and the mean position of the jet stream. In North America, the termination of the mid-Holocene dry interval was apparently time transgressive, beginning earlier in the northwest than in north-central North America; in the East, there currently are too few sites to clearly characterize late Holocene moisture patterns from lake records. We speculate that spatial variation in moisture increased in the late Holocene, perhaps as a result of the diminished seasonality of insolation and increased meridional flow. We also suggest that studies of highfrequency climate variation spanning different intervals of the Holocene are needed to investigate the interactions of large-scale boundary conditions, such as insolation and the ice sheet, with other climate forcing mechanisms in controlling spatial and temporal cli

Developing diatom-based transfer functions for Central Mexican lakes

This paper is the first attempt to produce diatom-based transfer functions for the northern tropical Americas. A dataset of 53 modern diatom samples and associated hydrochemical variables from 31 sites in the volcanic highlands of central Mexico is presented. The relationship between diatom species distribution and water chemistry is explored using canonical correspondence analysis (CCA) and partial CCA. Variance partitioning indicates that ionic strength and ion type both account for significant and independent portions of this variation. Transfer functions are developed for electrical conductivity (r2 = 0.91) and alkalinity (as a percentage of total anions) (r2 = 0.90), reflecting ionic strength and ionic composition respectively. Prediction errors, estimated using jack-knifing, are low for the conductivity model, but the carbonate transfer function performs less well. This study highlights the potential for diatom-based quantitative palaeoenvironmental reconstructions in central Mexico. However, a number of key diatom species found in fossil material are not represented in the modern flora. Sampling of additional sites may resolve this, but it is thought that the lack of modern analogues may reflect the high degree of anthropogenic disturbance in many of the catchments. This highlights the problem of trying to reconstruct pre-disturbance environmental changes in highly modified ecosystems. One possible solution is to merge the central Mexican data with the African dataset, which includes sites of similar chemical composition, but which have not suffered the same degree of disturbance.

Developing the Hull Acid Rain Model: Its Validation and Implications for Policy Makers.

The Hull Acid Rain Model (HARM) is extensively used in the UK to help in formulating strategies for reducing the emissions of acidifying pollutants. Here we describe the development of a 10 km version of HARM, which incorporates both software development and a new representation of the dry deposition of reduced N. Output from the new model for the UK is compared with S and N deposition data available from the UKs monitoring networks for 1992–1994 using linear regression. The model is able to reproduce the overall patterns of deposition for S, oxidised and reduced N. Although improved from earlier model versions, HARM still appears to underestimate dry deposition of reduced N. We describe the implications of the changes to HARM for assessing current policy commitments to reduce emissions of acidifying pollutants through changes in critical loads (CL) exceedance.

Sedimentary record of late-Holocene relative sea-level change and tectonic deformation from the Guerrero Seismic Gap, Mexican Pacific Coast

Studies of the coastal sedimentary record have allowed both the reconstruction of relative sea-level changes and the determination of local rates and magnitudes of tectonic deformation, particularly in tectonically active areas. Despite their successful use elsewhere, studies of this type are much less common for the Mexican Pacific coast, which parallels the Cocos-North America subduction plate boundary. Stratigraphic, geochemical and microfossil data from sediments in Laguna Mitla, the Pacific coast of Guerrero, Mexico, document late-Holocene sea-level changes induced by tectonic activity in the Mexican subduction zone. Three major events are identified. First, the formation of the lagoon by c. 4630 yr BP, as indicated by a freshwater to brackish peat. Second, a relative sea-level rise, or land subsidence, as indicated by a shift from a freshwater marginal lagoon environment to a marine setting, preceded by a marine inundation represented by a sand unit (possibly a tsunami deposit), by c. 3400 yr BP. And finally, a return to lagoonal conditions indicating a drop in relative sea level or coastal uplift by c. 2300 yr BP. The Laguna Mitla stratigraphy indicates general coastal subsidence or relative sea level rise of c. 1 mm/yr. We argue that these relative sea-level (land-level) changes have been induced by tectonic activity associated with the Mexican megathrust. A plausible explanation for the 3400 yr BP marine inundation is probably a tsunami produced by a large seismic event accompanied by coastal subsidence. Discrete fining upward, fine to coarse, sand units with an erosional basal contact, medium to poor sorting, and clay/mud rip-up clasts; an increase in Na and Sr elemental concentrations, indicative of a marine origin; and the landward extent of the sands support a tsunamigenic source for these deposits. However, these apparent tsunami deposits require further study to determine their lateral extent and to assess whether they can be correlated from one site to another. This study demonstrates the applicability of a multiproxy sedimentary approach in interpreting relative sea-level (land-level) changes and to derive data on related earthquake and tsunami events in tropical coastal lagoons.

The Holocene evolution of the Humber Estuary: reconstructing change in a dynamic environment

Abstract The Holocene sequence of the Humber Estuary displays a wide range of sediment types within which the preservation of microfossils is highly variable. Its evolution has been reconstructed using a range of environmental proxies with chronological control provided by more than 90 radiocarbon dates. Results are presented of diatom analyses from three cores typical of the inner, middle and outer estuary (HMB20, HMB7 and HMB12) and of three cores that illustrate the role of organic deposits (peats) and their associated pollen (HMB13, HMB12 and the Ancholme Valley) in the definition of sea-level index points. The reconstruction of relative sea-level change shows a rapid rise in the early Holocene, followed by a reduced rate of rise in the mid-late Holocene. This reconstruction, together with information on the pre-Holocene surface and the different palaeoenvironments from the cores have been integrated within a geographical information system and then interpreted to yield a series of palaeogeographical maps of the Humber at 1000-year time slices between 8 and 3 cal. ka bp. The marine transgression progressed up the estuary after 8 cal. ka bp, reaching the inner estuary by 6 cal. ka bp. The expansion of intertidal environments probably reached its maximum around 3 cal. ka bp. Changes since 3 cal. ka bp are described using archaeological and historical records. Tidal asymmetry is a major controlling factor on the balance of sediment accretion and erosion in the estuary. Sedimentary and bathymetric evidence suggests a damped oscillation between flood and ebb asymmetry in the Humber over the Holocene period. Such a conclusion would be of great importance to estuarine managers and users since it could be used to predict the future development of the estuary.

Tephrochronology of the Toluca Basin, central Mexico

Abstract The major element glass geochemistry of 10 Late Pleistocene and Holocene tephras found in the Toluca Basin in the Trans-Mexican Volcanic Belt is reported for the first time, as a key step towards establishing a tephrochronology for the region. The context for this tephrochronology is provided by a review of Late Quaternary volcanism of the basin. New tephra samples were collected from reference profiles around the basin in order to establish major element glass geochemical data for tephras described in previous publications, but not previously analysed. In addition, cores were taken from the basin floor to extend the known spatial distribution of the tephra layers and establish correlations with the lacustrine record. Improved chronological control has been provided by seven new AMS and conventional radiocarbon dates on the major tephras. Geochemical data are presented for 10 tephras ranging in age from ca. 25,000 BP (Lower Toluca Pumice) to about 8500 BP (Tres Cruces Tephra). Based on geochemical data, it is possible to correlate between sites within the Toluca Basin and between the Toluca Basin and the Basin of Mexico. The basis for a tephrochronological framework has been established.

The climate of Mexico since the Aztec period

Fluctuations in the climate of Mexico since the Aztec period (1345–1994) are established using a variety of primary and secondary historical sources. Drought conditions prevailed when the Aztecs entered the Basin of Mexico, but the climate became wetter shortly after this time and the period 1345–1640 appears to have been relatively wet. From 1640 to 1915 the country experienced a series of prolonged and devastating droughts which were extremely severe in the mid to late 1700s and the late 1800s. This interval coincides with periods of adverse weather conditions elsewhere in the world and may represent the period of the Little Ice Age. Since the early 1900s there has been a shift towards somewhat wetter conditions in Mexico. Variations in precipitation over this period can be explained by changes in the strength of the summer monsoon.