Paleoclimatic Signals Research Articles

Linear and nonlinear alignment of time series with applications to varve chronologies

AbstractThe problem of aligning time series arises naturally in a number of fields, but our investigation is motivated by the problem of extracting a paleoclimatic signal from glacial varve chronologies. Glacial varves are laminated sediments that record annual depositional cycles in certain lakes that are fed by glaciers. Varve thickness is determined by temperature and to a lesser extent by precipitation so that an accurate reconstruction of varve chronologies can be regarded as providing a potential long term proxy for paleoclimate. An important aspect of the varve problem is to align series from different locations. What makes the alignment problem particularly interesting is that in either series, one or more varves may be missing or a single varve might have been mistaken for a pair of varves.Traditionally, alignment problems have been treated using linear methods and concepts such as coherency and cross correlation. Since one of the pairs of varve series we look at is not well described by a linear model, in this paper we also employ nonlinear criteria that are derived from recent work on nonlinear identification and dependence modeling. More precisely, we use nonlinear least squares and measures of dependence such as the Kullback—Leibler measure and the Hellinger distance. Both local and global alignment are considered, and in particular we treat the situation where one or more observations may be missing from either series. Properties of the criteria are given and illustrated by simulation. We display examples where linear methods fail, but nonlinear criteria work. For the real data set of varves at our disposal, it will be seen that the nonlinear criteria perform as well as (or better than) the linear ones for the linearly related series, and that only the nonlinear criteria work for the two series that are not linearly related. Copyright © 2007 John Wiley & Sons, Ltd.

Late Quaternary paleoenvironments of an ephemeral wetland in North Dakota, USA: relative interactions of ground-water hydrology and climate change

This study of fossils (pollen, plant macrofossils, stomata and fish) and sediments (lithostratigraphy and geochemistry) from the Wendel site in North Dakota, USA, emphasizes the importance of considering ground-water hydrology when deciphering paleoclimate signals from lakes in postglacial landscapes. The Wendel site was a paleolake from about 11,500 14C yr BP to 11,100 14C yr BP. Afterwards, the lake-level lowered until it became a prairie marsh by 9,300 14C yr BP and finally, at 8,500 14C yr BP, an ephemeral wetland as it is today. Meanwhile, the vegetation changed from a white spruce parkland (11,500 to 10,500 14C yr BP) to deciduous parkland, followed by grassland at 9,300 14C yr BP. The pattern and timing of these aquatic and terrestrial changes are similar to coeval kettle lake records from adjacent uplands, providing a regional aridity signal. However, two local sources of ground water were identified from the fossil and geochemical data, which mediated atmospheric inputs to the Wendel basin. First, the paleolake received water from the melting of stagnant ice buried under local till for about 900 years after glacier recession. Later, Holocene droughts probably caused the lower-elevation Wendel site to capture the ground water of up-gradient lakes.

Review of recent developments in mineral magnetism of the Chinese loess

Mineral magnetism of the Chinese loess has been investigated for more than 20 years. Although there is a consensus that the neoformation of fine-grained maghemite particles in the superparamagnetic (SP) and single-domain (SD) grain size regions accounts for the magnetic enhancement in the Chinese paleosols, quantitative retrieval of paleoclimatic signals in terms of rock magnetic proxies is still a subject of debate. The ambiguities arise from the inherent complexities of magnetic proxies as well as the multiple factors that control the pedogenic processes. Therefore, a better description of the magnetic assemblage (including its mineralogy, grain size distribution and stoichiometry) of two distinct origins (pedogenic and eolian) can help us better understand mechanisms behind variations in magnetic proxies at different timescales, in order to link them to the paleoclimatic processes. This review focuses on recent developments in loess magnetism, and carefully evaluates merits and limitations of rock magnetic proxies. Furthermore, several currently unsolved problems are addressed.

Ground ice and slope sediments archiving late Quaternary paleoenvironment and paleoclimate signals at the margins of El'gygytgyn Impact Crater, NE Siberia

An accumulation terrace close to the El'gygytgyn Impact Crater in northeastern Siberia contains stratigraphic and periglacial evidence of the paleoenvironmental and paleoclimatic history and permafrost dynamics during late Quaternary time. A succession of paleo active-layer deposits that mirror environmental changes records periods favorable for the establishment and growth of ice-wedge polygonal networks and sediment variations. These two elements of the periglacial landscape serve as complementary paleoenvironmental archives that can be traced back to ∼ 14,000 cal yr BP. The slope sediments and the ground ice contained therein have prominent relative maxima and minima in properties (grain size, total organic content, oxygen isotopes). They document a regional early Holocene thermal maximum at about 9000 cal yr BP, followed by a transition to slightly cooler conditions, and a subsequent transition to slightly warmer conditions after about 4000 cal yr BP. Results from sedimentary analysis resemble morphological and geochemical (oxygen and hydrogen isotopes) results from ice wedge studies, in which successive generations of ice-wedge polygonal networks record warmer winters in late Holocene time. Moreover, peaks of light soluble cation contents and quartz-grain surface textures reveal distinct traces of cryogenic weathering. We propose a conclusive sedimentation model illustrating terrace formation in a permafrost terrain.

Recent Summer Climate Recorded in Complex Varved Sediments, Nicolay Lake, Cornwall Island, Nunavut, Canada

The recent (1950–1996) varve record from the proximal sediments in Nicolay Lake, Cornwall Island, Nunavut, Canada (77°46′ N, 94°40′ W) contains distinct subannual rhythmites. Deposition of these subannual rhythmites is due primarily to nival snow melt, with additional sedimentary units resulting from major summer precipitation and subaqueous mass wasting events. In order to evaluate the potential hydroclimatic signal contained in the varves from the unglacierized catchment, the nival deposition record was estimated by delineating the initial subannual rhythmite within each varve. When the record is split into temporal segments based on two phases that exhibit different sediment deposition patterns in the lake, the nival rhythmites are significantly correlated to annual cumulative melting degree days (MDD) from the nearest weather station Isachsen (78°47′ N, 103°32′ W) (1950–1962 AD and 1963–1977 AD with r = 0.55 and r = 0.82, respectively). A similar analysis with data from Resolute (74°43′ N, 94°59′ W) yields slightly weaker correlations (1950–1962 AD, r = 0.60; 1963–1994, r = 0.59). The strong positive correlation with both the Isachsen and Resolute thermal records suggests that the paleoclimatic signal in the sediments reflects regional climate conditions. Notably, the signal is strongest when the entire melt season is considered; weaker correlations with instrumental weather records are associated with comparisons limited to the peak melt or early season melt periods. We attribute this to the ongoing supply of snowmelt through the season in this polar region and the availability of sediment for transport throughout the melt season. These results indicate that a high resolution hydroclimatic signal is present in the sediments from Nicolay Lake and can be used for paleoclimate reconstruction provided sedimentary depositional controls are taken into account.

Quantifying grain size distribution of pedogenic magnetic particles in Chinese loess and its significance for pedogenesis

Quaternary glacial/interglacial cycles have been imprinted on the Chinese loess/paleosol sequences through pedogenesis. In order to accurately decode the paleoclimatic signals carried by these pedogenic particles it is essential to quantify the pedogenically produced magnetic particles in terms of mineralogy as well as grain size distribution (GSD). To date, the GSD has not been accurately determined because of the dearth of available means for analyzing extremely fine grained (nanometer‐scale) pedogenic magnetic particles. Using low‐temperature techniques, we systematically investigated the temperature dependency of χfd (defined as χ1Hz − χ10Hz, where χ1Hz and χ10Hz are AC magnetic susceptibility measured at 1 and 10 Hz, respectively) from two characteristic loess profiles, one located at the western Chinese Loess Plateau and the other in the central plateau. On the basis of Néel theory for a shape anisotropy dominant grain and experimental analysis at low temperatures, a quantitative GSD for pedogenic particles in Chinese loess/paleosols was constructed. We found that the dominant magnetic grain size lies just above the superparamagnetic/single‐domain threshold (∼20–25 nm) and that the GSD is almost independent of the degree of pedogenesis. This observation agrees well with other constraints from previous studies. This new GSD model improves our understanding of the pedogenic processes in Chinese loess, enabling further explicit linkage of environmental magnetism to paleoclimate changes.

Mineral magnetic variation of the Jiaodao Chinese loess/paleosol sequence and its bearing on long‐term climatic variability

To retrieve reliable long‐term paleoclimatic signals from the Chinese loess/paleosol sequences deposited over the past 2.6 Myr, multiparameter mineral magnetic investigations have been conducted on the Jiaodao section on the Chinese central Loess Plateau. First, the lithogenic magnetic components and the pedogenic overprinting were chemically separated by the citrate‐bicarbonate‐dithionite (CBD) method. The magnetic properties of the least pedogenically altered loess units and post‐CBD loess/paleosols are dominated by coarse‐grained lithogenic magnetite. Increasing degree of pedogenesis increases both the concentration and grain size of pedogenic magnetite/maghemite. Long‐term changes of the pre‐CBD magnetic properties suggest a long‐term decrease in summer monsoon intensity and a long‐term increase in winter monsoon intensity from the late Pliocene to the late Pleistocene. Long‐term variations of the post‐CBD magnetic properties suggest an increase in both concentration and grain size of coarse‐grained lithogenic PSD/MD magnetite grains, and hence indicate a long‐term increase in winter monsoon intensity over that period. The coercivity decrease of the lithogenic component for the loess unit L15 may be related to the accelerated uplift of the Tibetan Plateau and its adjacent regions during the period of L15 deposition. From paleosol S5 to loess L1, higher values of coercivity of the lithogenic component occur in typical loess and paleosols, and lower values are found at the loess/paleosol transition. These coercivity variations are thought to be related to significant changes in the source region as well as dust deposition in the Loess Plateau caused by changes in ice/snow or vegetational cover. These new results lead to a better understanding of the relationships between the effects of tectonic, paleoclimatic, and paleoenvironmental processes in northwestern China.

Mechanism of the magnetic susceptibility enhancements of the Chinese loess

Chinese loess/paleosol sequences have been regarded as excellent continental archives for encoding continuous paleoclimatic variations over the past 2.5 Myr. However, the mechanism for magnetic enhancements (especially the low‐field mass‐specific magnetic susceptibility, χ) of Chinese paleosols is still not completely resolved. This study quantifies contributions of aeolian and pedogenic magnetic particles to the bulk magnetic properties of the Chinese loess/paleosols by using a magnetic extraction technique. Magnetic properties of magnetic separates (extractable) and the corresponding residues (nonextractable) for five characteristic samples covering both loesses and paleosols were comprehensively investigated by hysteresis loops, frequency and low‐temperature dependence of magnetic susceptibility, and interparametric ratios. Results show that (1) with moderate degrees of pedogenesis (χ < 10 × 10−7 m3 kg−1), χ is enhanced more by increased concentration of stable single domain (SD) magnetic particles than by viscous superparamagnetic (SP) particles. For more mature paleosols with χ > (10–12) × 10−7 m3 kg−1, contributions of pedogenically related fine‐grained pseudosingle‐domain (PSD, ∼100 nm to several microns) particles become significant; (2) pedogenic particles have a narrow grain size distribution concentrated above the SP/SD threshold; and (3) anhysteretic remanent magnetization (ARM) is carried dominantly by SD grains. Moreover, we propose that only the nonextractable fraction of χ, saturation magnetization (Ms) and remanent magnetization (Mrs) show a strong relationship with the degree of pedogenesis. This new interpretation of magnetic enhancements helps us to retrieve more accurate and quantitative paleoclimatic signals recorded by the Chinese loess/paleosol sequences.

Seismic expression of Pleistocene paleoceanographic changes in the California Borderland from digitally acquired 3.5 kHz subbottom profiles and Ocean Drilling Program Leg 167 drilling

We correlate processed 3.5 kHz seismic profiles with physical properties of cores collected during ODP Leg 167 from the Tanner, East Cortes, and San Nicolas Basins through much of the Pleistocene succession. Results indicate that seismic horizons in the unconsolidated Pleistocene sediments (top 50 m) are mainly controlled by density contrasts. Removing of the compaction trend from the density reveals a very interesting relationship between density and composition ‐ the density closely and inversely correlates with organic carbon indicating that large‐scale variations in organic carbon are responsible for seismic reflections through their influence on density. This is a significant discovery since there apparently is no other paleoceanographic setting that we know of where such a close linkage between acoustic properties and organic carbon has been established. The variations in organic carbon are mainly marine in origin and derive from variations in primary productivity associated with upwelling and the preservation regime related to oxygenation of water. Pleistocene reflections on 3.5 kHz profiles in the Borderland province thus record regional cyclical fluctuations in the paleoclimatic signals. The close resemblance in the density profiles at the three different basins indicates that the sedimentary regime was similar in those basins through the Pleistocene. These common density patterns produce regional seismic horizons that correlate well among the basins. It is likely these correlated and dated horizons could be extrapolated to other Borderland basins (e.g., San Clemente), where they can potentially be used as time markers for neotectonic studies in the region.

Vostok (Antarctica) ice-core time-scale from datings of different origins

AbstractThree different approaches to ice-core age dating are employed to develop a depth–age relationship at Vostok, Antarctica: (1) correlating the ice-core isotope record to the geophysical metronome (Milankovich surface temperature cycles) inferred from the borehole temperature profile, (2) importing a known chronology from another (Devils Hole, Nevada, USA) paleoclimatic signal, and (3) direct ice-sheet flow modeling. Inverse Monte Carlo sampling is used to constrain the accumulation-rate reconstruction and ice-flow simulations in order to find the best-fit glaciological time-scale matched with the two other chronologies. The general uncertainty of the different age estimates varies from 2 to 6 kyr on average and reaches 6–15 kyr at maximum. Whatever the causes of this discrepancy might be, they are thought to be of different origins, and the age errors are assumed statistically independent. Thus, the average time-scale for the Vostok ice core down to 3350m depth is deduced consistent with all three dating procedures within the standard deviation limits of ±3.6 kyr, and its accuracy is estimated as 2.2 kyr on average. The constrained ice-sheet flow model allows, at least theoretically, extrapolation of the ice age–depth curve further to the boundary with the accreted lake ice where (at 3530m depth) the glacier-ice age may reach ∼2000 kyr.

Sublacustrine precipitation of hydrothermal silica in rift lakes: evidence from Lake Baringo, central Kenya Rift Valley

Abstract Many lakes in volcanic regions are fed by hot springs that, in some basins, can contribute a large percentage of the annual recharge, especially during times of aridity. It is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record. Hot spring deposits (travertine, sinter) provide the most tangible evidence for thermal recharge to lakes. Although subaerial spring deposits have been widely studied, lacustrine thermal spring deposits, especially sublacustrine siliceous sinters, remain poorly known. Detailed field, petrographic and scanning electron microscope (SEM) studies have been made of fossil sublacustrine sinter exposed at Soro hot springs along the northeastern shoreline of Ol Kokwe, a volcanic island in Lake Baringo, Kenya. Modern hot springs at Soro, which discharge Na–HCO3–Cl waters from a deep reservoir (∼180 °C ), have thin (1–10 mm), friable microbial silica crusts around their subaerial vents, but thicker (>1 cm) sinter deposits are not forming. The fossil sinter, which is present as intergranular cements and crusts in littoral conglomerates and sandstones, is composed mainly of opaline silica (opal-A). Three types of fossil sinter are recognized: (1) massive structureless silica, which fills intergranular pores and forms crusts up to 5 cm thick; (2) pore-lining silica, some of which is isopachous, and (3) laminated silica crusts, which formed mainly on the upper surfaces of detrital particles. All three types contain well-preserved diatoms including lacustrine planktonic forms. Microbial remains, mainly filamentous and coccoid bacteria (including cyanobacteria) and extracellular polymeric gels, are locally abundant in the opaline silica, together with detrital clays and thin laminae composed of authigenic chlorite (?). Most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water. Silica precipitated upon rapid cooling of thermal (∼90 °C ) waters at and just below their interface with the overlying cooler (∼25 °C ) lake waters. Microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids. Some of the silica originally may have been soft and partly gelatinous. Planktonic diatoms and detrital clay rained down, then became incorporated in the amorphous silica. Following a fall in lake level, the opal-A lithified and partly altered to cristobalite (opal-C) and chalcedony. The lowest fossil sinters were later encrusted by calcite stromatolites, with calcite and quartz forming late pore-filling cements. The age of the sublacustrine sinters is unknown, but some of the deposits could date back to the late Pleistocene. Similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring Lake Bogoria. Such rocks may provide evidence for deep, hot fluid recharge to lakes when encountered in the geological record.

Sedimentological and stratigraphic observations in El Mirón, a late Quaternary cave site in the Cantabrian Cordillera, Northern Spain

AbstractEl Mirón cave on the northern edge of the Cantabrian Cordillera is one of the first montane sites in this area to be the object of modern interdisciplinary research. Five excavation seasons have revealed an archaeological sequence attributable to the Medieval, Bronze, Chalcolithic, Neolithic, Mesolithic, Azilian, Magdalenian, and Solutrean periods, and early Upper and Middle Paleolithic levels have just been reached. Forty‐four radiocarbon ages range from 900 to 41,000 14C yr B.P. Sediments in the vestibule include sediment redeposited from an ancient colluvial fill in the inner cave, limestone rock fragments, biogenic CaCO3 generated by cryptogamic and vegetation growth on the cave ceiling, eolian silts, bird droppings, and anthropogenic lithic and faunal remains. Sedimentological analyses of the cave fill reported here document the transition from a sandy silt facies with rounded cobbles in the inner vestibule to a carbonate‐rich silt facies with angular limestone clasts near the cave mouth. There is no clear regional paleoclimatic signal in the bulk sediment analyses because of the heavy influence of local input and human disturbance. Micromorphology by M.‐A. Courty (see Courty and Vallverdu [this issue]; and also Courty et al. [1989]) reveals subtle evidence for some short‐term variations that can be correlated with regional paleoclimates, such as the Younger Dryas event. © 2001 John Wiley & Sons, Inc.

Comparison of cosmogenic radionuclide production and geomagnetic field intensity over the last 200 000 years

The production rate of cosmogenic radionuclides such as 10Be or 14C is known to vary as a function of the geomagnetic field intensity. It should, therefore, be possible to extract a record of palaeofield intensity from the deposition record of these radionuclides in marine or terrestrial sediments and ice cores. Field intensity variations, however, are not the only factor that has influenced the cosmogenic radionuclide records. In the case of 14C, variations of the global carbon cycle, caused by reorganization of the ocean circulation patterns from the last glacial to the present interglacial, are superimposed. 10Be is not affected by these variations because it is not part of the carbon cycle, but its deposition rates in marine sediments vary as a function of lateral sediment redistribution and boundary scavenging intensity. A global stacked record of 10Be deposition rates, corrected for sediment redistribution by normalizing to 230Thex, was shown to remove most of the disturbances, and provides a record of 10Be production rate variations over the last 200 000 years, which translates into geomagnetic field intensity variations. This dataset is compared with palaeofield intensities reconstructed from marine sediments by palaeomagnetic methods, from variations in atmospheric 14C/12C derived from independent calibrations of 14C ages, such as U/Th dating and tree ring chronology, and from 36Cl and 10Be fluxes in polar ice cores. Potential influences of the Earth’s orbital parameters and insufficient correction for orbitally triggered climate variations on the palaeointensity reconstructions are assessed. It is argued that the palaeointensity records derived from marine sediments are not significantly affected by these factors.

Unraveling climatic changes from intraprofile variation in oxygen and hydrogen isotopic composition of goethite and kaolinite in laterites: an integrated study from Yaou, French Guiana

An integrated study of O and H isotopes in the lateritic profile of Yaou, French Guiana, was undertaken to investigate the usefulness of stable isotopes as tracers of climatic changes in continental environments. The studied profile is composed of a 27 m thick saprolite, mostly developed in the past under wet-and-dry tropical climate in association with a duricrust, overlain by a 3 m thick yellow latosol formed more recently under present equatorial hot and humid climate. δ 18O–δD values determined for weathering goethite (pseudomorphs after pyrite) and kaolinite (microcrystalline clay groundmass) throughout the 30 m deep profile reflect formation temperatures consistent with present (25°C) and realistic past climatic temperatures (20°C–30°C), indicating that weathering minerals formed in isotopic equilibrium with their genetic environment and were not subjected to significant isotope exchange after formation. A distinct shift downward (2‰ for δ 18O, 15‰ for δD) from low to high δ 18O–δD values occurs around 20 m depth in the saprolite. It is interpreted as recording the change from the past tropical to the present equatorial climate. Goethite and kaolinite in the 5–10 m thick saprolite interval immediately above the active basement weathering front are in isotopic equilibrium with modern water and must have formed under present equatorial-humid conditions. In contrast, goethite and kaolinite found higher up in the saprolite and in the duricrust formed in the past under tropical wet and dry climate from waters distinctly depleted in 18O and D relative to modern water. The marked depletion of paleo-meteoric water at Yaou most likely reflects a more contrasted or “monsoonal” character of the ancient tropical climate. The present study shows that ancient weathering minerals in lateritic profiles preserve their δ 18O–δD values and carry a time signal. The time signal is best expressed in minerals formed rapidly at the weathering front and not subjected to post-formational remobilization, such as the goethite pseudomorphs after pyrite occurring at Yaou. Groundmass kaolinite is more susceptible to partial remobilization, through successive dissolution-precipitation reactions, which may obliterate the paleoclimatic signal. Unraveling the climatic record carried by weathering minerals in old soil systems is greatly enhanced by studying both oxygen and hydrogen isotopic compositions.

Jurassic (Toarcian to Kimmeridgian) dinoflagellate cysts and paleoclimates

Multivariate statistics have been used to identify the paleoclimatic‐paleoecological affinities of most European Jurassic (Toarcian to Kimmeridgian) dinoflagellate cyst taxa. These analyses were then used to construct dinoflagellate cyst diagrams (abbreviated to ‘cyst diagrams'), which are analogous to pollen diagrams. The marine and terrestrial records were then correlated using both pollen (miospore) and cyst diagrams. These plots appear to include many biostratigraphically significant paleoclimatic events which augment, and in some cases refine, the ammonite‐based biostratigraphy. The integration of the terrestrial spore‐pollen based correlations with the oceans, via dinoflagellate cysts, appears to prove that the oscillations in the palynomorph diagrams reflect widespread temperature changes. This is because the ‘cold’ group of dinoflagellate cysts includes several taxa with north‐polar paleogeographic distributions, and equivalent fluctuations can be matched in the terrestrial spore‐pollen diagrams, which in turn are correlated between the northern and southern hemispheres. In the early mid Jurassic, warming events were noted above the Toarcian‐Aalenian boundary and in the early Bajocian. The paleoclimatic signals in the Bathonian appear to be difficult to interpret consistently. The Callovian‐Oxfordian boundary is characterized by a marked cooling event. The Late Jurassic following the early Oxfordian appears to have been a time of increasing ambient paleotemperatures with a thermal zenith in the Kimmeridgian. This generally rising paleotemperature trend for the Callovian to Kimmeridgian interval, including specific variations, is consistent with other Jurassic paleoclimatic results derived using different methods such as clay mineral analyses, isotopic methods, miospore eco‐groups and macropaleobotany.

Monitoring the regional and temporal variability of winter snowfall in the arid Andes using digital NOAA/AVHRR data

This paper deals with the spatial distribution and the temporal variability of snowfall in the most arid part of the Andes (18°‐ 28°S) during southern hemisphere winter (May‐September). As the official precipitation data is of poor quality, analyses were carried out by means of digital image processing techniques, using NOAA/AVHRR satellite‐data. Through analysis of 24 different snowfall events from six winters, a previously unknown spatial and temporal precipitation pattern in this remote and unexplored area was revealed. Snowfall is most abundant in the southernmost part of the research area and on the western side of the Andes, indicating the Pacific origin of the snowfall. Nevertheless, the typical snowfall pattern is modified during different periods of the winter. Three typical time periods could be defined and distinguished from one another. Each of these three periods is characterized by typical weather conditions (cold fronts and “cut‐offs “) leading to a distinct snowfall pattern. As this study is part of a broader paleoclimatic project, the results will serve as a basis for paleoclimatic reconstruction of past climate. Only by knowing the modern circulation and precipitation patterns is it possible to interpret paleoclimatic signals and archives found in the study area (e.g. paleosol, moraines) correctly.

Temperature history and accumulation timing for the snowpack at GISP2, central Greenland

AbstractPrevious research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated thatδD andδ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of theδDrecord at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2data. The slope of the multi-yearTvsδcorrelation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.

Temperature history and accumulation timing for the snowpack at GISP2, central Greenland

AbstractPrevious research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated that δD and δ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of the δD record at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2 data. The slope of the multi-year T vs δ correlation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.

The thermal regime of the crystalline continental crust: Implications from the KTB

An extensive geothermal research program within the German Continental Deep Drilling Program (KTB) covered an almost complete spectrum of experimental and theoretical aspects. The main results and conclusions are as follows: (1) Equilibrium temperature is 118.6°C at 4000 m in the KTB pilot borehole (KTB‐VB) and will be around 260°C at 9100 m in the KTB main borehole (KTB‐HB). Time required for thermal equilibration of the KTB‐HB to within 1% of the initial perturbation will be about 13–16 years. (2) The failure to predict temperature correctly for the KTB was mainly due to an unaccounted perturbation by a transient ground surface temperature history. (3) Pleistocene surface temperature variations affect the present‐day crustal temperature between 1.3 to 2.7 K up to a depth of 4000 m. Accordingly, present‐day heat flow density is systematically too low down to approximately 1500 m. Model simulations indicate that groundwater flow does not eliminate paleoclimatic signals, even though it may translate them to a depth incompatible with both their diffusive age and their amplitude. These results emphasize the importance of an adequate consideration of paleoclimatic effects for the interpretation of thermal data. (4) Lateral heat transport is significant when steep inclination and folding of the rock formations coincide with contrasts in thermal conductivity. This is indicated by typical variations in the vertical components of temperature gradient and heat flow density, such as in the KTB‐HB. In contrast, thermally relevant advection of heat is confined to the top 500–1000 m. In the vincinity of the KTB, this is about twice the maximum difference in elevation. (5) In the deeper crust, free convection systems require permeabilities greater than 10−17 m2 for large rock volumes, but simple numerical models indicate that the associated temperature regimes are imcompatible with KTB borehole data. (6) Heat production rate shows no systematic variation with depth and is related to lithology at the KTB as in other deep boreholes in crystalline rock. Numerical models, using heat production rate derived from seismic velocities, yield temperatures compatible with KTB borehole data.

The Taconite Inlet Lakes Project: a systems approach to paleoclimatic reconstruction

A comprehensive study of meteorological, hydrological, limnological and sedimentological conditions in the watersheds of density-stratified (meromictic) lakes around Taconite Inlet, Northern Ellesmere Island, N.W.T., Canada was carried out from 1990–1992. Lakes C1 and C2 contain seawater ‘trapped’ by isostatic uplift as the former embayments became isolated from the sea. These lakes, and Lake C3, contain varved sediments which provide an annually resolvable paleoclimatic record. By studing the major systems influencing sedimentation in one of these lakes (Lake C2) a better understanding of the climatic controls on varve formation, and hence on the paleoclimatic signal in the varved sediment record, was obtained. The varves of Lake C2 provide a proxy record of summer temperature for the region.