Stable carbon and oxygen isotope record od IODP Hole 306-U1313

The Mid-Pleistocene transition (MPT) was the time when quasi-periodic (? 100 kyr), high-amplitude glacial variability developed in the absence of any significant change in the character of orbital forcing, leading to the establishment of the characteristic pattern of late Pleistocene climate variability. It has long been known that the interval around 900 ka stands out as a critical point of the MPT, when major glaciations started occurring most notably in the northern hemisphere. Here we examine the record of climatic conditions during this significant interval, using high-resolution stable isotope records from benthic and planktonic foraminifera from a sediment core in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313). We have considered the time interval from late in Marine Isotope Stage (MIS) 23 to MIS 20 (910 to 790 ka). Our data indicate that interglacial MIS 21 was a climatically unstable period and was broken into four interstadial periods, which have been identified and correlated across the North Atlantic region. These extra peaks tend to contradict previous studies that interpreted the MIS 21 variability as consisting essentially of a linear response to cyclical changes in orbital parameters. Cooling events in the surface record during MIS 21 were associated with low benthic carbon isotope excursions, suggesting a coupling between surface temperature changes and the strength of the Atlantic meridional overturning circulation. Time series analysis performed on the whole interval indicates that benthic and planktonic oxygen isotopes have significant concentrations of spectral power centered on periods of 10.7 kyr and 6 kyr, which is in agreement with the second and forth harmonic of precession. The excellent correspondence between the foraminifera d18O records and insolation variations at the Equator in March and September suggests that a mechanism related to low-latitude precession variations, advected to the high latitudes by tropical convective processes, might have generated such a response. This scenario accounts for the presence of oscillations at frequencies equal to precession harmonics at Site U1313, as well as the occurrence of higher amplitude oscillations between the MIS22/21 transition and most of MIS 21, times of enhanced insolation variability.

Stable carbon and oxygen isotope record of IODP Site 306-U1313, MIS 23-19

Since the seminal work by Hays et al. (1976), a plethora of studies has demonstrated a correlation between orbital variations and climatic change. However, information on how changes in orbital boundary conditions affected the frequency and amplitude of millennial-scale climate variability is still fragmentary. The Marine Isotope Stage (MIS) 19, an interglacial centred at around 785 ka, provides an opportunity to pursue this question and test the hypothesis that the long-term processes set up the boundary conditions within which the short-term processes operate. Similarly to the current interglacial, MIS 19 is characterised by a minimum of the 400-kyr eccentricity cycle, subdued amplitude of precessional changes, and small amplitude variations in insolation. Here we examine the record of climatic conditions during MIS 19 using high-resolution stable isotope records from benthic and planktonic foraminifera from a sedimentary sequence in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313) in order to assess the stability and duration of this interglacial, and evaluate the climate system's response in the millennial band to known orbitally induced insolation changes. Benthic and planktonic foraminiferal d18O values indicate relatively stable conditions during the peak warmth of MIS 19, but sea-surface and deep-water reconstructions start diverging during the transition towards the glacial MIS 18, when large, cold excursions disrupt the surface waters whereas low amplitude millennial scale fluctuations persist in the deep waters as recorded by the oxygen isotope signal. The glacial inception occurred at ~779 ka, in agreement with an increased abundance of tetra-unsaturated alkenones, reflecting the influence of icebergs and associated meltwater pulses and high-latitude waters at the study site. After having combined the new results with previous data from the same site, and using a variety of time series analysis techniques, we evaluate the evolution of millennial climate variability in response to changing orbital boundary conditions during the Early-Middle Pleistocene. Suborbital variability in both surface- and deep-water records is mainly concentrated at a period of ~11 kyr and, additionally, at ~5.8 and ~3.9 kyr in the deep ocean; these periods are equal to harmonics of precession band oscillations. The fact that the response at the 11 kyr period increased over the same interval during which the amplitude of the response to the precessional cycle increased supports the notion that most of the variance in the 11 kyr band in the sedimentary record is nonlinearly transferred from precession band oscillations. Considering that these periodicities are important features in the equatorial and intertropical insolation, these observations are in line with the view that the low-latitude regions play an important role in the response of the climate system to the astronomical forcing. We conclude that the effect of the orbitally induced insolation is of fundamental importance in regulating the timing and amplitude of millennial scale climate variability.

Stable isotope record of benthic foraminifera from the Gulf of California

A high-resolution, accelerator radiocarbon dated climate record of the interval 8,000-18,000 years B.P. from Deep Sea Drilling Project site 480 (Guaymas Basin, Gulf of California) shows geochemical and lithological oscillations of oceanographic and climatic significance during deglaciation. Nonlaminated sediments are associated with cooler climatic conditions during the late glacial (up to 13,000 years B.P.), and from 10,300 to 10,800 years B.P., equivalent to the Younger Dryas event of the North Atlantic region. We propose that the changes from laminated (varved) to nonlaminated sediments resulted from increased oxygen content in Pacific intermediate waters during the glacial and the Younger Dryas episodes, and that the forcing for the latter event was global in scope. Prominent events of low delta18O are recorded in benthic foraminifera from 8,000 to 10,000 and at 12,000 years B.P.; evidence for an earlier event between 13,500 and 15,000 years B.P. is weaker. Maximum delta18O is found to have occurred 10,500, 13,500, and 15,000 years ago (and beyond). Oxygen isotopic variability most likely reflects changing temperature and salinity characteristics of Pacific waters of intermediate depth during deglaciation or environmental changes within the Gulf of California region. Several lines of evidence suggest that during deglaciation the climate of the American southwest was marked by increased precipitation that could have lowered salinity in the Gulf of California. Recent modelling studies show that cooling of the Gulf of Mexico due to glacial meltwater injection, which is believed to have occurred at least twice during deglaciation, would have resulted in increased precipitation with respect to evaporation in the American southwest during summertime. The timing of deglacial events in the Gulf of Mexico and the Gulf of California supports such an atmospheric teleconnection.

Mineralogy of ODP Leg 104 holes

The mineralogical and geochemical study of samples from Sites 642, 643, and 644 enabled us to reconstruct several aspects of the Cenozoic paleoenvironmental evolution (namely volcanism, climate, hydrology) south of the Norwegian Sea and correlate it with evolution trends in the northeast Atlantic. Weathering products of early Paleogene volcanic material at Rockall Plateau, over the Faeroe-Iceland Ridge and the Voring Plateau indicate a hot and moist climate (lateritic environment) existed then. From Eocene to Oligocene, mineralogical assemblages of terrigenous sediments suggest the existence of a warm but somewhat less moist climate at that time than during the early Paleogene. At the beginning of early Miocene, climatic conditions were warm and damp. The large amounts of amorphous silica in Miocene sediment could indicate an important flux of silica from the continent then, or suggest the formation of upwelling. Uppermost lower Miocene and middle to upper Miocene clay assemblages suggest progressive cooling of the climate from warm to temperate at that time. At the end of early Miocene, hydrological exchanges between the North Atlantic and the Norwegian Sea became intense and gave rise to an important change in the mineralogy of deposits. From Pliocene to Pleistocene, the variable mineralogy of deposits reflects alternating glacial/interglacial climatic episodes, a phenomenon observed throughout the North Atlantic.

Pollen record from sediment core M77/2_056-3 (Bay of Guayaquil, Peru)

We analyzed the pollen content of a marine core located near the bay of Guayaquil in Ecuador to document the link between sea surface temperatures (SST) and changes in rainfall regimes on the adjacent continent during the Holocene. Based on the expansion/regression of five vegetation types, we observe three successive climatic patterns. In the first phase, between 11,700 and 7700 cal yr BP, the presence of a cloud (Andean) forest in the mid altitudes and mangroves in the estuary of the Guayas Basin, were associated with a maximum in boreal summer insolation, a northernmost position of the Intertropical Convergence Zone (ITCZ), a land- sea thermal contrast, and dryness. Between 7700 and 2850 cal yr BP, the expansion of the coastal herbs and the regression of the mangrove indicate a drier climate with weak ITCZ and low ENSO variability while austral winter insolation gradually increased. The interval between 4200 and 2850 cal yr BP was marked by the coolest and driest climatic conditions of the Holocene due to the weak influence of the ITCZ and a strengthening of the Humboldt Current. After 2850 cal yr BP, high variability and amplitude of the Andean forest changes occurred when ENSO frequency and amplitude increased, indicating high variability in land-sea connections. The ITCZ reached the latitude of Guayaquil only after 2500 cal yr BP inducing the bimodal precipitation regime we observe today. Our study shows that besides insolation, the ITCZ position and ENSO frequency, changes in eastern equatorial Pacific SSTs play a major role in determining the composition of the ecosystems and the hydrological cycle of the Ecuadorian Pacific coast and the Western Cordillera in Ecuador.

Pollen and macrofossils profiles and age determination from the Pittsburg Basin in the lowlands of south-central Illinois

Pollen and macrofossil evidence for the nature of the vegetation during glacial and interglacial periods in the regions south of the Wisconsinan ice margin is still very scarce. Modern opinions concerning these problems are therefore predominantly derived from geological evidence only or are extrapolated from pollen studies of late Wisconsinan deposits. Now for the first time pollen and macrofossil analyses are available from south-central Illinois covering the Holocene, the entire Wisconsinan, and most probably also Sangamonian and late Illinoian time. The cores studied came from three lakes, which originated as kettle holes in glacial drift of Illinoian age near Vandalia, Fayette County. The Wisconsinan ice sheet approached the sites from the the north to within about 60 km distance only. One of the profiles (Pittsburg Basin) probably reaches back to the late Illinoian (zone 1), which was characterized by forests with much Picea. Zone 2, most likely of Sangamonian age, represents a period of species-rich deciduous forests, which must have been similar to the ones that thrive today south and southeast of the prairie peninsula. During the entire Wisconsinan (14C dates ranging from 38,000 to 21,000 BP) thermophilous deciduous trees like Quercus, Carya, and Ulmus occurred in the region, although temporarily accompanied by tree genera with a more northerly modern distribution, such as Picea, which entered and then left south-central Illinois during the Woodfordian. Thus it is evident that arctic climatic conditions did not prevail in the lowlands of south-central Illinois (about 38°30' lat) during the Wisconsinan, even at the time of the maximum glaciation, the Woodfordian. The Wisconsinan was, however, not a period of continuous forest. The pollen assemblages of zone 3 (Altonian) indicate prairie with stands of trees, and in zone 4 the relatively abundant Artemisia pollen indicates the existence of open vegetation and stands of deciduous trees, Picea, and Pinus. True tundra may have existed north of the sites, but if so its pollen rain apparently is marked by pollen from nearby stands of trees. After the disappearance of Pinus and Picea at about 14,000 BP (estimated!), there developed a mosaic of prairies and stands of Quercus, Carya, and other deciduous tree genera (zone 5). This type of vegetation persisted until it was destroyed by cultivation during the 19th and 20th century. Major vegetational changes are not indicated in the pollen diagram for the late Wisconsinan and the Holocene. The dating of zones 1 and 2 is problematical because the sediments are beyond the14C range and because of the lack of stratigraphic evidence. The zones dated as Illinoian and Sangamonian could also represent just a Wisconsinan stadial and interstadial. This possibility, however, seems to be contradicted by the late glacial and interglacial character of the forest vegetation of that time.