Jason H. Curtis

Correlation of Late Miocene to Early Pliocene sequences between the Mediterranean and North Atlantic

Ocean Drilling Program (ODP) Site 982 in the North Atlantic contains a complete latest Miocene to early Pliocene section that was tuned to the astronomical timescale by correlating the record of gamma ray attenuation (GRA) bulk density to summer insolation at 65°N and the benthic δ18O signal to orbital obliquity for the interval from 4.6 to 7.5 Ma. The astronomical tuning of the Site 982 record permits a direct bed-to-bed correlation to the cyclostratigraphy of Messinian sections in the Mediterranean [Krijgsman et al., 1999a, 2001]. The benthic δ18O signal at Site 982 records a latest Miocene glacial period that lasted from ∼6.26 to 5.50 Ma and consisted of 18 glacial-to-interglacial oscillations that were controlled by the 41-kyr cycle of obliquity. Although the intensification of glaciation at 6.26 Ma may have contributed to the restriction of the Mediterranean, it preceded the depositional onset of the lower evaporite unit at 5.96 Ma by some 300 kyr. The transition from Stage TG12 to TG11 at 5.5 Ma marks the end of the latest Miocene glacial period and precedes the Miocene/Pliocene boundary by 170 kyr. Although benthic δ18O values are relatively low and δ18O of bulk carbonate reaches a minimum at the Miocene/Pliocene boundary at 5.33 Ma, there is no single “event” that would indicate deglaciation and sea level rise as the cause of the reflooding of the Mediterranean. We conclude that glacioeustatic changes alone were not responsible for either the start or end of evaporite deposition during the Messinian, suggesting that tectonic or local climate changes in the Mediterranean region were the dominant cause(s).

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

We explored the use of carbon and nitrogen isotopes (δ13C and δ15N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four 210Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and 226Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low δ13C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher δ13C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the δ15N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both δ13C and δ15N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that δ13C and δ15N reflect shifts in historic lake trophic state.

Impact of iceberg melting on Mediterranean thermohaline circulation during Heinrich events

Down-core samples of planktonic and benthic foraminifera were analyzed for oxygen and carbon isotopes in International Marine Past Global Changes Study (IMAGES) core MD99-2343 in order to study the interactions between climate change in the Northern Hemisphere and the western Mediterranean thermohaline circulation at times of Heinrich events (HE). Our results confirm the antiphase correlation between enhanced North Atlantic Deep Water formation and low ventilation in the Mediterranean. However, this study reveals that this antiphase relationship in deepwater formation between the North Atlantic and Mediterranean was interrupted during times of HE when the injection of large volumes of water from melting icebergs reached the entrance to the Mediterranean. These events, which lasted less than 1000 years, are represented by pronounced decreases in both planktonic d18O and benthic d13C signals. Lower salinities of Mediterranean surface water resulted in a slowdown of western Mediterranean deepwater overturn even though cold sea surface temperatures and drier climate should have resulted in enhanced deepwater formation.

A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten, Guatemala

We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS 14C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to ∼9000 14C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by ∼9000 14C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (∼9000 to ∼6800 14C yr BP), pollen data suggest moist conditions, but high δ 18O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lakes water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in δ 18O values and associated change in ostracod species at ∼6800 14C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning ∼5780 14C yr BP may indicate early human disturbance. By ∼2800 14C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning ∼1100 to 1000 14C yr BP, following the collapse of Classic Maya civilization.

A Holocene vegetation history from lowland Guatemala

A 5.45-m core from Lake Peten-Itza, lowland Guatemala, contains a near-complete record of Holo cene sedimentation. The age-depth relationship for the core is based on AMS 14C dating of terrestrial wood fragments and provides a reliable chronology in this karst region where hard-water lake error has typically confounded sediment geochronology. In the basal part of the sequence, pollen of the Moraceae-Urticaceae group dominate, indicating the presence of widespread tropical forest during the early Holocene (c. 8600-5600 yr BP). Relative abundance of pollen of high forest taxa declined beginning as early as 5600 yr BP, indicating climatic drying or perhaps initial land clearance. Deforestation by prehistoric Mayan inhabitants is documented clearly in the pollen record beginning about 2000 yr BP (106 cal BC-122 cal AD, 95.4%, 2 sigmas) by the appearance of disturbance taxa (e.g. Ambrosia and Poaceae) and presence of Zea. Forest regrowth occurred following the Classic Maya collapse, c. AD 900, as reflected by a relative increase in Moraceae-Urticaceae pollen.

Holocene climate variability in the western Mediterranean region from a deepwater sediment record

[1] The detailed analysis of the International Marine Past Global Changes Study core MD99-2343 recovered from a sediment drift at 2391 m water depth north of the island of Minorca illustrates the effects of climate variability on thermohaline circulation in the western Mediterranean during the last 12 kyr. Geochemical ratios associated with terrigenous input resulted in the identification of four phases representing different climatic and deepwater overturning conditions in the Western Mediterranean Basin during the Holocene. Superimposed on the general trend, eight centennial- to millennial-scale abrupt events appear consistently in both grain size and geochemical records, which supports the occurrence of episodes of deepwater overturning reinforcement in the Western Mediterranean Basin. The observed periodicity for these abrupt events is in agreement with the previously defined Holocene cooling events of the North Atlantic region, thus supporting a strong AtlanticMediterranean climatic link at high-frequency time intervals during the last 12 kyr. The rapid response of the Mediterranean thermohaline circulation to climate change in the North Atlantic stresses the importance of atmospheric teleconnections in transferring climate variability from high latitudes to midlatitudes.

Orbital and internal forcing of climate on the Yucatan Peninsula for the past ca. 36 ka

Abstract A 19.6 m sedimentary sequence from Lake Quexil, Guatemala, is presented as a proxy record of climatic change on the low-lying Yucatan Peninsula over the last ca. 36 ka. Long-term climatic fluctuations are attributed to orbital forcing of insolation, while abrupt changes during the late glacial period are the result of internal forcing mechanisms that remain imperfectly modelled. GCMs simulating meltwater cooling of the Gulf of Mexico may predict some but not all climatic responses in the Circum-Caribbean. Lake Quexil did not experience greater aridity than predicted by orbital forcing during late glacial zones Q-LG 1 and 2, which were drier only in comparison with Holocene conditions. Q-LG 1 appears to have been colder than expected based on insolation alone, and is contemporary with the Younger Dryas event between meltwater pulses 1A and 1B. Thus, other feedbacks also may have cooled surface temperatures in the Gulf of Mexico, such as unique late glacial circulation patterns, increased albedo for the Florida and Yucatan Peninsulas, or surface temperatures on the North American continent. Additional terrestrial sequences are needed to improve the chronology and confirm the distribution of sub-regional climatic variability in the Circum-Caribbean.

PALEOLIMNOLOGY OF THE MAYA LOWLANDS Long-term perspectives on interactions among climate, environment, and humans

Since the late 1950s, scientists have used sediment cores from lakes on the Yucatan Peninsula to explore the complex interactions among climate, environment, and ancient Maya culture. Early paleolimnological studies generally assumed that late Holocene climate was invariable. Consequently, paleolimnologically inferred environmental changes that occurred during the past 3,000 years or so—for example, forest decline and soil erosion—were attributed wholly to anthropogenic activities such as land clearance for agriculture and construction. Recent high-resolution, proxy-based paleoclimate records from continental and insular sites around the Caribbean Sea contradict the assumption of late Holocene climate stability. Instead, these core data suggest that regional drying began about 3,000 years ago and that the past three millennia were characterized by variable moisture availability. Paleoclimate inferences from Lakes Chichancanab and Punta Laguna, northern Yucatan Peninsula, indicate that drought events over the past 2,600 years were cyclical. These dry events, thought to have been driven by solar forcing, appear to have occurred approximately every two centuries (about 208 years). The driest period of the late Holocene occurred between A.D. 800 and 1000, coincident with the Classic Maya Collapse. We review the history of paleolimnological studies in the Maya Lowlands, discuss the difficulty of differentiating climatic signals from anthropogenic signals in late Holocene lake sediment profiles, and assess current understanding of past climate changes in the region based on regional lacustrine sediment studies.

Holocene climatic and human influences on lakes of the Yucatan Peninsula, Mexico: an interdisciplinary, palaeolimnological approach

We used palaeolimnological techniques to examine effects of Holocene climate change and human influence on lakes of the Yucatan Peninsula, Mexico. The three study lakes are located along a west-east transect that represents a gradient of increasing modern precipitation and density of former Maya settlement. At Lake Coba, an 880-cm sediment core yields a complete record of lacustrine sedimentation that began when the lake first filled ∼8000 BP as groundwater level rose in response to rising sea level and increased precipi tation. Diatom, ostracod, and δ 18O evidence indicate that Lake Coba was initially shallow and saline. Coba, presently in the region of greatest rainfall, showed more episodic water-level changes than Lake San Jose Chulchaca or Lake Sayaucil. High lake level and fresh water were evident at 440 cm (∼2600 BP), followed by a decline in water levels and an increase in total ionic salinity to the present time. In a 613-cm core from Sayaucil, in the intermediate precipitation zone, total salinity was high between 600 and 400 cm (∼3050 and 2000 BP), followed by consistently higher water levels. Salinity was high in the lower portion of a 110-cm San Jose Chulchaca core (beginning ∼1860 BP), followed by a gradual and consistent freshening of water to the present time. Trophic state changes and human influence on lakes were evaluated using diatom, δ13C, total P, sedimentary organic matter, and preliminary pollen data. Maximal human disturbance at Lake Coba, a densely settled Maya urban site, occurred during a deep-water event at 440 cm, followed by a decline in human influence and trophic state to the present time. Trophic state and linear sedimentation rates in Sayaucil increased significantly above 400 cm (after ∼2000 BP), probably associated with initial Maya settlement near Xtojil and subsequent small- scale farming. Limnological disturbance may have preceded the period of maximal human occupation because initial land clearance and consequent soil erosion probably affected water quality substantially. San Jose Chul chaca lacks archaeological evidence of human occupation in the drainage, and shows gradual changes in trophic state not caused by human disturbance. With the exception of a 14C date on wood from the base of the Coba core, 14C dates and chronologies may be artificially old as a consequence of hard-water-lake error. Trophic state changes in the study lakes were generally consistent with known patterns of human settlement and popu lation change. Late-Holocene water-level fluctuations were most pronounced in Lake Coba in eastern Yucatan, where modern rainfall is currently greatest, but lake level is generally lower than during much of the past. Lake level was relatively constant in Sayaucil in the central peninsula, whereas lake level in San Jose Chulchaca in the arid western portion of the peninsula increased gradually over time.

Climate change in the Lake Valencia Basin, Venezuela, ∼12600 yr BP to present

We present a palaeoclimate record for northern South America, extending from the latest Pleistocene (~12600 14C yr BP) to present. Climate reconstruction for the Valencia Basin, Venezuela, was based on sediment geochemistry and δ18O records from ostracod and gastropod shells in a 568 cm sediment core. Sediment chronology was established by AMS 14C dating of terrestrial wood fragments. From ~12600 to ~10000 14C yr BP the Valencia basin was drier than present and the coring site, now under 9.4 m of water, was only intermittently wet. After ~10000 14C yr BP, moisture availability increased and lake level rose, permanently covering the core site. From ~10000 to ~8200 14C yr BP, Lake Valencia was hydrologically closed and the isotope records reflect pronounced variability in the ratio of evaporation to precipitation. During the wetter early to middle Holocene (~8200 to ~3000 14C yr BP), lake level was high and water was lost to outflow. Greater moisture availability at this time may have been caused by increased intensity of the annual cycle (with wetter wet seasons and drier dry seasons), a result of large, orbitally driven differences in seasonal insolation. Two brief periods of lower lake level, at ~7000 and ~3300 14C yr BP, are indicated in the oxygen isotope and calcium carbonate records. Since ~3000 14C yr BP, water level in Lake Valencia has been dropping.