PP34A-01 INVITED
Tropical Pacific Hydrologic Changes During the Last 1,000 Years from Lipid D/H Ratios in Lake and Ocean Sediments
The climate of the last millennium was punctuated by two prominent events: the Medieval Warm Period (MWP) from 800-1300 A.D. and the Little Ice Age (LIA) from 1400-1850. The impact of, and evidence for these events derives almost exclusively from the middle latitudes of the Northern Hemisphere continents, largely from tree rings and mountain glaciers. Remarkably little attention has been given to the tropical climate of the last millennium and its potential impact on the middle and high latitudes even though massive fluxes of latent heat, moisture and momentum derive from there. Indeed, proxy records of the ENSO system, the monsoons of Asia and India, and the position of the Intertropical Convergence Zone (ITCZ) indicate that the last 1,000 years were a time of profound change in the tropical climate globally. Here we present evidence from D/H ratios in lipids from lake and ocean sediments in the western, central and eastern tropical Pacific that the LIA was a time of marked change in rainfall patterns. Specifically, our data (1) strongly imply that the ITCZ was poleward of 5 degrees north latitude for at least 2,500 years prior to 1600 A.D., and (2) are consistent with a frequency and/or intensity of El Niņo that was high during the Little Ice Age compared to the rest of the last millennium.
PP34A-02 INVITED
An Eight-Century High-Resolution Paleoclimate Record From the Cariaco Basin: Baseline Variability and the 20th Century
The Cariaco Basin, Venezuela is well-positioned to record a detailed history of surface ocean changes along the southern margin of the Caribbean and the tropical Atlantic. Varved, high deposition rate sediments deposited under anoxic conditions and an abundance of well-preserved microfossils result in one of the few marine records capable of preserving evidence of interannual- to decadal-scale climate variability in the tropical Atlantic. Here we present Mg/Ca and stable oxygen isotope data with sub-decadal resolution derived from sediments deposited over the last 800 years. Mg/Ca measured on the planktic foraminifer Globigerina bulloides from a Cariaco Basin sediment core strongly correlates with spring (March-May) instrumental SSTs between AD 1870 and 1990. The long-term record displays a surprising amount of variability for a tropical location. The temperature swings are not necessarily related to local upwelling variability, but instead represent wider conditions in the Caribbean and western tropical Atlantic. The Mg/Ca-SST record also captures the decadal and multidecadal variability observed in global land and sea surface temperature anomalies, and correlates with Atlantic tropical storm and hurricane frequency over the late-19th and 20th centuries. On average, 20th century temperatures are not the warmest in the entire record, but they do show the largest increase in magnitude and fastest rate of SST change over the last eight hundred years. Stable oxygen isotope data also correlate well with instrumental SSTs, but not over the full instrumental record. Poor correlations with early instrumental SST data suggest a salinity overprint. However, reconstructing δ- water variability using combined Mg/Ca and δ18O data is not straightforward as the δ- water/salinity relationship varies seasonally in the Cariaco Basin. Comparisons with percent titanium data suggest intervals of both local and regional surface salinity changes over the length of the record.
PP34A-03
Modern Stable Isotope Systematics in Heshang Cave, China: Implications for Speleothem Based Paleoclimate Reconstructions
Stable isotope (δ18O and δ13C) records from speleothems collected in Heshang Cave, China (30.44°N, 110.42°E) contain detailed information about past variability in East Asian monsoon rainfall. In particular, HS4, a 2.5 m long, annually laminated stalagmite, which grew continuously since the early Holocene until it was collected in 2001, contains seasonal variations in δ18O and δ13C (and trace elements) which may provide information about past seasonal and inter-annual monsoon variability. To investigate the modern environmental controls on seasonal geochemical variations preserved in Heshang Cave speleothems, an intensive environmental and hydrochemical monitoring program was initiated in late 2003. Here we present stable isotope data (δD and δ18O) from cave drip waters collected at two locations every ten days during this period. In addition, we present δ18O and δ13C results from calcite precipitated on glass slides in monthly intervals over a two year period. This data suggests that modern calcite is forming in isotopic equilibrium in Heshang Cave and that seasonal variations in speleothem δ18O are caused by both seasonal variations in drip water composition, which are closely related to rainfall, and seasonal temperature variations within Heshang Cave, which are closely related to outside temperature. The most negative speleothem δ18O values are observed during the peak of the summer monsoon, reflecting the low rainfall δ18O (because of the amount effect) and high cave temperatures at this time. We expect this relationship to remain constant in the past, and thus δ18O can serve as a useful time marker to tie other seasonal geochemical variations to the annual cycle. Seasonal δ13C variations in Heshang Cave speleothems are most likely reflecting changes in the fraction of CO2 degassing that occurs either above the cave as prior calcite precipitation or on the speleothem surface. The highest δ13C values occur during the summer monsoon season, suggesting perhaps that temperature, growth rate, and drip rate, which all peak at this time, may play a role. We hope to deconvolve these controls using a multi-proxy approach which includes trace element, in addition to stable isotope analysis.
PP34A-04 INVITED
Working at the margins with sessile proxy archives: Different considerations for anthropogenic-scale reconstructions
On the time scale most applicable to questions of anthropogenic climate change (centuries to millennia), considerations regarding reconstructions of temperature and salinity differ from those necessary on glacial/deglacial time scales. These different considerations involve the assumptions that 1. significant changes in the salinity/oxygen isotope relationship may have occurred during the record, 2. recorded changes are indicative of open ocean conditions, and 3. global scale change is best measured in well defined climatic zones. The first assumption is required in glacial/interglacial work due to the large volume of meltwater and resultant change in the isotopic value of the fresh water end-member. On the anthropogenic time scale, one can assume a constant relationship between oxygen isotopes and salinity in most locations, eliminating one source of propagated error. The second assumption is often made in glacial/deglacial studies, as well as other studies using proxy records archived in sediments, because sediment constituents live in the water column of the open oceans. Anthropogenic-scale salinity and temperature reconstructions often rely on massive accretionary skeletons of organisms such as corals and sclerosponges which grow in proximity to land masses that support sessile organisms. The effects of landmasses cannot be underestimated - data from the island of Roatan, Honduras, show the affects of local land use patterns on oxygen isotope ratios, one of the key proxies for temperature and salinity reconstructions. Additionally, sclerosponge salinity reconstructions from the Bahamas corroborate coral reconstructions across the Atlantic basin at the decadal scale, however local effects preclude reproducibility on high-frequency time scales. Care must be taken to apply these data to open ocean interpretations. Finally, subtle changes in ocean circulation at the anthropogenic time scale are best observed in proxy records from the margins of water masses rather than the cores of well-defined water masses or climatic zones. Here, large changes resulting in heave or lateral movement of a water mass can be mistaken for unbelievable trends in a single water mass. Cycles observed in the cores of single water masses may yield to "trends" near the margins of the same water masses due to long- period changes in local and regional circulation patterns. Ultimately, anthropogenic-scale temperature and salinity reconstructions are more precise at the first order, but prescription of either regional or marginal reconstructions to global, open ocean changes involves different assumptions than reconstructions at the glacial/deglacial time scale.
PP34A-05
Independent Temperature and Salinity Reconstruction from Coral Skeleton
Massive coral skeleton offers the best-suited material for reconstruction of tropical climate during the last century. Indeed, growth rate of some species is fast enough to provide high-resolution (monthly) sampling. The formation of a big colony may cover continuously several decades, even, several centuries. Chronology is made easy by annual growth layers. Finally, aragonite, which composes this skeleton, presents several proxies, such as isotopes and trace elements. However, the aragonite deposit is biologically controlled and all the proxies are influenced by both environmental and biologic factors. By example, temperature and light may affect both oxygen isotopes as well as Sr/Ca. Thus, these two parameters are difficult to separate from seasonal records. Such an effect may be neglected for annual averages or filtered records. Indeed, the difference of irradiation recorded during two consecutive years is much limited than between winter and summer (see PP24). It is the reason that annual and monthly reconstitutions will be conducted separately. In addition, it seems that other factors are the causes of the high variability shown by all the proxies. The common factor affecting them is related with metabolism. We suppose that proxies being measured from a powder collected from homogeneous material are fractionated by external factors through the same biologic filter during the time, but each of them differently because incorporated in the mineral by different ways. This is the reason that we used neural network (NN), which learns the behavior of several proxies submitted to one forcing during a known period. Then, the complex relationship recognized by neurons between the different proxies is used to "predict" the forcing during the past. The relationship between external factor and proxies remains hidden, but this could not be used for other colonies, even for other sampling from a same head. By this way, it is possible to calibrate temperature and salinity and thus to reconstruct separately, with two different networks, the two variables and thus reduce errors. A similar operation may be performed for seasonal variations. Results obtained from a coral core harvested in Fiji from seven proxies for SST and four for SSS, indicated that SST and SSS are modified at each strong El Nino during the last century. The optimization of the use of this statistical treatment and the estimation of the error will be discussed.
PP34A-06
Coral Records of Late 20th Century Warming and Freshening in the Central Tropical Pacific
Tropical Pacific climate variability strongly impacts global temperature and rainfall patterns. The evolution of tropical Pacific climate under anthropogenic greenhouse warming is unclear, observational and modeling studies support both a strengthening (Cane et al.,1997) and a weakening (Vecchi et al.,2006) of the tropical Pacific zonal sea-surface temperature (SST) gradient. Corals from the central tropical Pacific (CTP) have provided monthly-resolved climate proxy records that contain late 20th century trends toward more negative oxygen isotopic (d18O) values, suggesting that regional warming and/or freshening has occurred over the last several decades (Urban et al.,2000; Cobb et al.,2001). It is important to identify the physical mechanisms that underlie these trends in order to better predict low-frequency changes in tropical Pacific climate over the next decades. One potential mechanism involves a reduction in the tropical Pacific zonal SST gradient, which would warm the CTP by reducing upwelling while increasing rainfall (thereby decreasing salinity). On the other hand, warming might have occurred absent any change in the zonal SST gradient, and the freshening may signal changes in the location and/or intensity of the Inter-Tropical Convergence Zone (ITCZ). Here we apply d18O and Sr/Ca proxies to modern corals from Palmyra, Fanning and Christmas Islands (2-6°N,157-162°W) in order to reconstruct SST and sea-surface salinity (SSS) over the last several decades. While coral d18O changes in SST and/or the d18O of seawater (d18Osw, which is linearly related to salinity in this region), coral Sr/Ca is mainly controlled by SST. When used together, these two proxies allow us to quantify recent climate trends in the CTP with respect to SST and SSS, particularly because Palmyra, Fanning and Christmas span strong gradients in SST and SSS. The islands are aligned in a NW-SE fashion, such that Christmas (2°N) is bathed by the South Equatorial Current and dominated by upwelling variability; while Palmyra (6°N) lies in the core of the North Equatorial Counter Current (NECC) and is heavily influenced by ITCZ variability. We measure coral Sr/Ca ratios with an analytical error of better than ±0.3percent or ~±0.4°C(1σ). We measure coral d18O values with an analytical error of ±0.05percent(1σ). After converting the coral Sr/Ca measurements into SST anomalies, we construct d18Osw timeseries by removing the Sr/Ca-derived SST contribution from the coral d18O. The Sr/Ca-based SST reconstructions show that Palmyra and Christmas have warmed by 0.6 and 1.5°C over the period 1970-1998, respectively. The stronger warming trend at Christmas is consistent with a reduction in equatorial upwelling over this period. The d18Osw timeseries suggest that Palmyra and Christmas have experienced seawater freshening of 0.94psu and 0.36psu, respectively over the period. The stronger freshening trend at Palmyra may suggest enhanced and/or a southward migration of the ITCZ, and/or a reduction in the salinity of waters advected from the West Pacific Warm Pool by the NECC. Preliminary SST and SSS reconstructions from Fanning, which lies between Palmyra and Christmas, confirm these late 20th century warming and freshening trends. Taken together, the coral-based evidence for simultaneous warming and freshening in the CTP is consistent with a weakening of the tropical Pacific zonal SST gradient in the late 20th century. References: Cane et al.(1997).Science,275:957-960. Cobb et al.(2001).Geophys.Res.Lett.,28(11):2209-2212. Urban et al.(2000).Nature,407:989-993. Vecchi et al.(2006).Nature,441:73-76.
PP34A-07
Seasonality recorded in Modern and Viking Limpet Shells ( Patella vulgata), Quoygrew, Orkney, UK
Climate archives contained in shells of the European limpet, Patella vulgata, from Viking shell middens can potentially provide much needed information about seasonality in mid-latitude coastal areas prior to the complicating effects of industrialization. P. vulgata shells are common in the stratified middens accumulated by the Viking inhabitants of Quoygrew, Orkney, and were likely used for baiting fish. Radiocarbon dates and artifacts place these middens between the 9th/10th and 13th centuries. This interval coincides with the Medieval Warm Period. Little is known about the seasonal temperature variation during this time of pre-industrial warming. Before reconstructing climate information from Viking shells, we determined whether P. vulgata preserves environmental and ecological information. Previous work on live-collected specimens from Whitley Bay near Newcastle-upon-Tyne, England, confirmed that: (1) oxygen isotope ratios served as a proxy for sea surface temperature after accounting for a uniform +1.01 +/-0.21 ‰ offset; and (2) annual growth lines occurred during the winter given this location is within the cold-temperature biogeographic province. Winter growth lines and increments are common growth patterns found in marine bivalves from the cold-temperate province along the western North Atlantic. Preliminary isotope data from the 9th/10th century reveals similar winter and summer temperature relative to today and annual growth lines formed during winter, typical of a cold-temperate habitat.
PP34A-08
Simulation of spaceborne salinity observation of the world oceans
Surface salinity needs to be measured with an accuracy of 0.1-0.2 psu over 100-300km at 7-30day sampling interval, to monitor important open ocean phenomena such as North Atlantic thermohaline circulation, ocean surface freshwater balance, surface ocean stability in the western tropical Pacific Ocean, and halosteric effect on sea level. To address the need, a spaceborne L-band microwave radiometer, Aquarius, is under development by NASA for launch in 2010. The accuracy requirement of Aquarius mission is set to 0.2 psu over 100km for monthly averaged salinity observations. To examine the feasibility of the accuracy goal, we developed an end-to-end simulator incorporating surface emission model, radiative transfer model for the Earth and cosmic radiation, ionospheric model, and antenna model. The retrieval simulation corrects for these forward processes to estimate the surface salinity. The result shows that surface salinity may be retrieved with an accuracy better than 0.2 psu rms over the global open ocean (warm ocean with surface temperature higher than 10C, at least 500km away from the coast, and at 100km spatial resolution). The retrieval error increases toward colder seas.