Isotopic and Geochemical Constraints on Paleoclimate Processes I
Presiding: T van de Flierdt, Lamont-Doherty Earth Observatory; S R Hemming, Lamont-Doherty Earth Observatory
PP21A-01 08:30h
Covariation of Precipitation Δ18O and Relative Humidity and the Implication for Changing the Atmospheric Circulation Pattern between Glacial and Interglacial Periods
It has been demonstrated that the oxygen and hydrogen isotopic compositions of tree rings contain information about the isotopic composition of the source water and the relative air humidity during photosynthesis. More recently, Shu et al. showed that the slope of the ΔD vs. Δ18O plot contains information about covariation between isotopic composition of the source water (or approximately precipitation) and the relative humidity of the atmosphere. If the variation of the precipitation Δ18O or ΔD is independent of the variation in the prevailing relative humidity, the ΔD vs. Δ18O plot of tree rings should have the same slope as that of the meteoric water line. If, on the other hand, the precipitation Δ18O is positively (or negatively) correlated with the relative humidity, the ΔD vs. Δ18O plot of tree rings would have a greater (or smaller) slope than that of the meteoric water line. Shu et al. demonstrated a positive precipitation Δ18O - relative humidity covariation using cellulose analyses from trees across a precipitation gradient in the Olympic Mountains, Washington, USA. Here we use this concept to compare the ΔD vs. Δ18O relationship in Holocene samples with those from the last glaciation in the mid-latitude continental United States. The data are separated into three groups, Holocene (age < 10 kyr), Glacial (age > 14 kyr) and Transition (10 kyr < age < 14 kyr). We found that during the Holocene the ΔD vs. Δ18O relationship has a slope of 13.7±2.3, significantly greater than the slope of the global meteoric water line of 8. For the last glaciation, the slope is 5.3±0.9, and the slope for the transition period is 4±1.2, both significantly smaller than 8. One possible explanation is a northward extension of the north boundary of the Hadley cell in the northern hemisphere, which would have caused mid-latitude precipitation to be deep convective in nature, producing the amount effect similar to that of low-latitude precipitation today. These results may imply a general circulation pattern in glacial times different from that of interglacial times.
PP21A-02 08:45h
Causes for Large-Scale Precipitation Variability in the Southwestern USA During the Late Holocene
The continental climate variability of the late Holocene can be difficult to evaluate for lack of high-resolution, dateable proxies. We have compiled an annually resolved record of relative moisture for the last 3000 years using stalagmites in southeastern New Mexico. Stalagmite growth bands were combined with high-precision U-Th dates to develop the high-resolution record, with wetter periods indicated by intervals of thicker banding, and drier periods by thinner banding, hiatuses, and changes in mineralogy. Although the response of individual stalagmites to wet/dry events varies, comparisons to the regional instrumental precipitation and tree ring data confirm that the stalagmites reflect regional moisture variability. The compiled stalagmite record exhibits dramatic variability over the past 3000 years, motivating our study into what may be driving precipitation variability in the study region. We compared the modern instrumental precipitation record for the region with the Southern Oscillation Index (SOI) and Pacific Decadal Oscillation (PDO) Index and found significant correlation, suggesting the El Nino-Southern Oscillation (ENSO) and PDO are possible mechanisms for modulating past precipitation variability. To test this hypothesis, we conducted time series analyses on contemporaneous segments of two stalagmite records with no apparent hiatuses to assess any similarities in the spectral densities. Multiple-taper spectral analysis was used to test for significant narrowband or harmonic signals and wavelet analysis conducted to evaluate the amplitude and frequency modulation of the spectral peaks in the time series. Results show statistically significant peaks at both interannual (2-5 years) and decadal-scale (15-30, 40-50 years) frequencies. While interannual variability is present throughout the evolutive spectrum, it is the decadal-scale variability that coincides with major shifts and overall increases in precipitation in both records. Given the modern precipitation regime, it is likely the decadal events correspond to the PDO or a low-frequency component of ENSO (or ENSO-like mechanism). The timing of these peaks in the evolutive spectrum suggests that these mechanisms have been driving the moisture variability in the Southwest over the past 3000 years.
PP21A-03 09:00h
A New Look at the Global Distribution of Isotopes in Precipitation
The hydrogen and oxygen isotopic ratios of meteoric water, as preserved in geologic materials, are widely used to reconstruct past climatic conditions. Correlation between temperature and isotopic composition of precipitation for temperate regions has been well documented, and modern day correlations are used in interpreting paleo-isotopic records, although temperature is not the direct causative factor in isotopic distribution. Semi-tropical and tropical locations demonstrate correlation between amount of precipitation and isotopic value. In this report, we propose that the parameter (E-P)/P, where E is evaporation and P is precipitation, explains the annual mean isotopic distribution in both temperate and tropical environments. This parameter is also shown to correlate with the seasonal distribution of isotopic ratios in precipitation globally. Global data was downloaded from the GNIP (Global Network for Isotopes in Precipitation) database (http://isohis.iaea.org). This set is comprised of monthly hydrogen and oxygen isotopic compositions, amount of precipitation, and temperature for over 500 stations throughout the world. The monthly oxygen isotopic data were plotted against station latitude and a spline function fitted for each month. The resulting distributions show a local minimum in isotopic values in the tropics, which migrates seasonally following the ITCZ. On either side of the minimum, two asymmetric maxima occur which change in position and amplitude throughout the year. We use a simple one-dimensional model to predict the seasonal isotopic distribution. The model divides the world into 5-degree latitudinal zones. Seasonally varying evaporation and precipitation are specified for each zone from marine climatologic records, and the model uses the mass balance equations to calculate the import or export of water vapor and the isotopic value of precipitation falling in each zone. The results show that the major variations in isotopic value of precipitation at all latitudes can be explained by this simple model, the precipitation Δ18O being positively correlated with (E-P)/P. When (E-P)/P is more negative, more of the precipitated vapor must be "imported" ± from a distance and is thus isotopically depleted, and when (E-P)/P is positive, the precipitated vapor is the early fraction of the locally evaporated water to precipitate and is isotopically enriched.
PP21A-04 09:15h
New Stable Isotope Tropical Paleoclimate Proxies
Organized tropical rain systems such as tropical cyclones (TC) and mesoscale convective systems (MCS) produce both water vapor and rainfall with distinctly low isotope ratios. This lowering is caused by recyling of water in organized systems. Therefore, fresh water carbonate organisms have considerable potential to act as proxy recorders of these systems. Ostracoda are ephemeral making them especially attractive candidates. Tropical trees offer another opportunity because the low isotopic spikes produced in both soil waters when heavy rains result and ambient water vapor surronding the trees may be recorded in the tree cellulose. Ostracoda living in the surface waters derived from Tropical Storm Allison (2001) document the passage of the storm in their oxygen isotope ratios. The stable isotopic composition of water vapor along the southwest coast of Mexico shows considerable variation in response to TC and MCS activity offshore even when no rain falls in the region. Potentially a long-term record of this activity may be found in the stable isotopic composition of trees providing low elevation trees of sufficient longevity can be found.
PP21A-05 09:30h
Oxygen Isotope and Mg/Ca Constraints on the East Pacific Equatorial Front From the LGM to the Holocene
Ocean-atmosphere interactions in the eastern tropical Pacific produce and maintain a pronounced asymmetry in sea surface temperature (SST) distribution about the equator. The equatorial cold tongue exhibits a southerly bias with minimum SSTs centered near 2°S, while the Intertropical Convergence Zone (ITCZ) has a strong northerly bias with a mean position near 7°N. The equatorial cold tongue-ITCZ front is today a strong diagnostic of ocean-atmosphere coupling and its strength is correlated with upwelling intensity and ITCZ position. Here we provide new constraints on the variability of this front from the Last Glacial Maximum to the Holocene. Oxygen isotope and Mg/Ca ratios of the planktonic foraminifer G. ruber were measured in two cores from opposite sites of the front: V21-30 in the cold tongue (1.2°S) and V28-134 in the northern Cocos Ridge (6.9°N). Both proxies indicate an enhancement of the cross-equatorial front during deglaciation and establishment of a stronger SST gradient in the Holocene compared to the LGM. Average LGM surface cooling of 2°C is observed in the cold tongue site, and 3-3.5°C in the Cocos Ridge site. These results support the recently proposed model that latitudinal shifts of the Pacific ITCZ (southward during glacial periods) modulate the strength of tropical SST gradients on orbital timescales, and can potentially exert an important influence on the character of ENSO variability.
PP21A-06 INVITED 09:45h
Variability in surface and thermocline temperatures of the Eastern Equatorial Pacific during the last glacial period
Here we present results from ODP Core Site 1240 (Leg 202, 0 ° 01.31'N, 86° 27.76'W, 2921 mbsl) located in the Eastern Equatorial Pacific (EEP), under the northern edge of the so-called "Cold Tongue". This frontal system separates the cold, salty waters of the Peru Current from warmer and fresher tropical waters of the Northern Hemisphere. This study is based on paired stable isotopes (Δ18O, Δ13C) and trace elements (Mg/Ca) from planktonic foraminifera. Multiple species with different dwelling preferential depths were studied to reconstruct both surface and thermocline characteristics. Core-top Mg/Ca inferred temperatures are in good agreement with current mean annual sea surface temperature (SST) and deep thermocline temperatures (DTT) as recorded from available data from hydrographical stations. Our reconstruction covers the last glacial period at centennial-time resolution. Age model has been constructed in base to eight AMS 14C dates for the last 20 kyr interval. The older section of Site 1240 has been dated after visual correlation of the Δ18O record measured in the deep thermocline dwelling foraminifer N. dutertrei and the Deuterium record from the Antarctic. The resemblance of both records is very remarkable, as is confirmed by the high correlation coefficient (R=0.92) estimated for the tied interval. Results reveal the occurrence of several rapid oscillations of about 1-2° C are observed during MIS 3 in both, SST (G. ruber) and DTT (N. dutertrei) records. Apparently, most of the Heinrich Events correlate with short warming phases although they are not particularly large within the record. N. dutertrei record indicates a long term shallowing of the thermocline across the glacial period and the occurrence of some millennial events of deepening. These events correlate with intervals of relatively high salinity in the surface layer. These changes are likely to reflect short term oscillations in the ITCZ in relation to high latitude millennial-scale oscillations.