Stable isotope ratios and chemistry on corals from the Caribbean Sea

Instrumental climate data are limited in length and only available with low spatial coverage before the middle of the 20th century. This is too short to reliably determine and interpret decadal and longer scale climate variability and to understand the underlying mechanisms with sufficient accuracy. A proper knowledge of past variability of the climate system is needed to assess the anthropogenic impact on climate and ecosystems, and also important with regard to long-range climate forecasting. Highly-resolved records of past climate variations that extend beyond pre-industrial times can significantly help to understand long-term climate changes and trends. Indirect information on past environmental and climatic conditions can be deduced from climate-sensitive proxies. Large colonies of massive growing tropical reef corals have been proven to sensitively monitor changes in ambient seawater. Rapid skeletal growth, typically ranging between several millimeters to centimeters per year, allows the development of proxy records at sub-seasonal resolution. Stable oxygen isotopic composition and trace elemental ratios incorporated in the aragonitic coral skeleton can reveal a detailed history of past environmental conditions, e.g., sea surface temperature (SST). In general, coral-based reconstructions from the tropical Atlantic region have lagged behind the extensive work published using coral records from the Indian and Pacific Oceans. Difficulties in the analysis of previously utilized coral archives from the Atlantic, typically corals of the genera Montastrea and Siderastrea, have so far exacerbated the production of long-term high-resolution proxy records. The objective of this study is the evaluation of massive fast-growing corals of the species Diploria strigosa as a new marine archive for climate reconstructions from the tropical Atlantic region. For this purpose, coral records from two study sites in the eastern Caribbean Sea (Guadeloupe, Lesser Antilles; and Archipelago Los Roques, Venezuela) were examined. At Guadeloupe, a century-long monthly resolved multi-proxy coral record was generated. Results present the first d18O (Sr/Ca)-SST calibration equations for the Atlantic braincoral Diploria strigosa, that are robust and consistent with previously published values using other coral species from different regions. Both proxies reflect local variability of SST on a sub-seasonal scale, which is a precondition for studying seasonally phase-locked climate variations, as well as track variability on a larger spatial scale (i.e., in the Caribbean and tropical North Atlantic). Coral Sr/Ca reliably records local annual to interannual temperature variations and is higher correlated to in-situ air temperature than to grid-SST. The warming calculated from coral Sr/Ca is concurrent with the strong surface temperature increase at the study site during the past decades. Proxy data show a close relationship to major climate signals from the tropical Pacific and North Atlantic (the El Niño Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO)) affecting the seasonal cycle of SST in the North Tropical Atlantic (NTA). Coral oxygen isotopes are also influenced by seawater d18O (d18Osw) which is linked to the hydrological cycle, and capture large-scale climate variability in the NTA region better than Sr/Ca. Results from a quantitative comparison between extreme events in the two most prominent modes of external forcing, namely the ENSO and NAO, and respective events recorded in seasonal coral d18O imply that SST variability at the study site is highly linked to Pacific and North Atlantic variability, by this means supporting the assumptions of observational- and model-based studies which suggest a strong impact of ENSO and NAO forcings onto the NTA region through a modulation of trade wind strength in winter. Results from different spectral analysis tools suggest that interannual climate variability recorded by the coral proxies is II largely dictated by Pacific ENSO forcing, whereas at decadal and longer timescales the influence of the NAO is dominan. tThe Archipelago Los Roques is situated in the southeastern Caribbean Sea, north of the Venezuelan coast. Year-to-year variations in monthly resolved coral d18O of a nearcentury- long Diploria strigosa record are significantly correlated with SST and show pronounced multidecadal variations. About half of the variance in coral d18O can be explained by variations in seawater d18O, which can be estimated by calculating the d18Oresidual via subtracting the SST component from measured coral d18O. The d18Oresidual and a regional precipitation index are highly correlated at low frequencies, suggesting that d18Osw variations are primarily atmospheric-driven. Warmer SSTs at Los Roques broadly coincide with higher precipitation in the southeastern Caribbean at multidecadal time scales, effectively strengthening the climate signal in the coral d18O record. The Los Roques coral d18O record displays a strong and statistically significant relationship to different indices of hurricane activity during the peak of the Atlantic hurricane season in boreal summer and is a particularly good indicator of decadal-multidecadal swings in the latter indices. In general, the detection of long-term changes and trends in Atlantic hurricane activity is hampered due to the limited length of the reliable instrumental record and the known inhomogeneity in the observational databases which result from changes in observing practice and technology over the years. The results suggest that coral-derived proxy data from Los Roques can be used to infer changes in past hurricane activity on timescales that extend well beyond the reliable record. In addition, the coral record exhibits a clear negative trend superimposed on the decadal to multidecadal cycles, indicating a significant warming and freshening of surface waters in the genesis region of tropical cyclones during the past decades. The presented coral d18O time series provides the first and, so far, longest continuous coral-based record of hurricane activity. It appears that the combination of both signals (SST and d18Osw) in coral d18O leads to an amplification of large-scale climate signals in the record, and makes coral d18O even a better proxy for hurricane activity than SST alone. Atlantic hurricane activity naturally exhibits strong multidecadal variations that are associated with the Atlantic Multidecadal Oscillation (AMO), the major mode of lowfrequency variability in the North Atlantic Ocean. However, the mechanisms underlying this multidecadal variability remain controversial, primarily because of the limited instrumental record. The Los Roques coral d18O displays strong multidecadal variability with a period of approximately 60 years that is closely related to the AMO, making the Archipelago Los Roques a very sensitive location for studying low-frequency climate variability in the Atlantic Ocean. In summary, the coral records presented in this thesis capture different key climate variables in the north tropical Atlantic region very well, indicating that fast-growing Diploria strigosa corals represent a promising marine archive for further proxy-based reconstructions of past climate variability on a range of time scales.

Stable-isotope records from Dronning Maud Land, Antarctica

The European Project for Ice Coring in Antarctica (EPICA) includes a comprehensive pre-site survey on the inland ice plateau of Dronning Maud Land. This paper focuses on the investigation of the 18O content of shallow firn and ice cores. These cores were dated by profiles derived from dielectric-profiling and continuous flow analysis measurements. The individual records were stacked in order to obtain composite chronologies of 18O contents and accumulation rates with enhanced signal-to-noise variance ratios.These chronologies document variations in the last 200 and 1000 years.The 18O contents and accumulation rates decreased in the 19th century and increased during the 20th century.Using the empirical relationships between stable isotopes, accumulation rates and the 10m firn temperature, the variation of both parameters can be explained by the same temperature history.But other causes for these variations, such as the build-up of the snow cover, cannot be excluded. A marked feature of the 1000 year chronology occurs during the period AD 1180-1530 when the 18O contents remains below the long-term mean. Cross-correlation analyses between five cores from the Weddell Sea region and Dronning Maud Land show that 18O records can in some periods be positively correlated and in others negatively correlated, indicating a complex climatic history in time and space.

Annual layer thicknesses and age-depth (oldest estimate) of Derwael Ice Rise (IC12), Dronning Maud Land, East Antarctica

A 120 m-long ice core was drilled in 2012 on the Derwael Ice Rise, coastal Dronning Maud Land, East Antarctica. Water stable isotopes (d18O and dD) stratigraphy is supplemented by discontinuous major ion profiles and continuous electrical conductivity measurements. The base of the ice core is dated to AD 1759 ± 16, providing a climate proxy for the past ~250 years. This data set presents the core's annual layer thickness history in meters water equivalent for the oldest age-depth estimate before correction for the influence of ice deformation.

Sr/Ca ratios and oxygen isotopes from sclerosponges

We investigate aragonitic skeletons of the Caribbean sclerosponge Ceratoporella nicholsoni from Jamaica, 20 m below sea level (mbsl), and Pedro Bank, 125 mbsl. We use d18O and Sr/Ca ratios as temperature proxies to reconstruct the Caribbean mixed layer and thermocline temperature history since 1400 A.D. with a decadal time resolution. Our age models are based on U/Th dating and locating of the radiocarbon bomb spike. The modern temperature difference between the two sites is used to tentatively calibrate the C. nicholsoni Sr/Ca thermometer. The resulting calibration points to a temperature sensitivity of Sr/Ca in C. nicholsoni aragonite of about -0.1 mmol/mol/K. Our Sr/Ca records reveal a pronounced warming from the early 19th to the late 20th century, both at 20 and 125 mbsl. Two temperature minima in the shallow water record during the late 17th and early 19th century correspond to the Maunder and Dalton sunspot minima, respectively. Another major cooling occurred in the late 16th century and is not correlatable with a sunspot minimum. The temperature contrast between the two sites decreased from the 14th century to a minimum in the late 17th century and subsequently increased to modern values in the early 19th century. This is interpreted as a long-term deepening and subsequent shoaling of the Caribbean thermocline. The major trends of the Sr/Ca records are reproduced in both specimens but hardly reflected in the d18O records.

Vegetation cover and radiocarbon dates of palsa and peat plateaus in the Hudson Bay Lowlands

The Holocene development of a treed palsa bog and a peat plateau bog, located near the railroad to Churchill in the Hudson Bay Lowlands of northeastern Manitoba, was traced using peat macrofossil and radiocarbon analyses. Both sites first developed as wet rich fens through paludification of forested uplands around 6800 cal. yr BP. Results show a 20th-century age for the palsa formation and repeated periods of permafrost aggradation and collapse at the peat plateau site during the late Holocene. This timing of permafrost dynamics corroborates well with that inferred from previous studies on other permafrost peatlands in the same region. The developmental history of the palsa and peat plateau bogs is similar to that of adjacent permafrost-free fens, except for the specific frost heave and collapse features associated with permafrost dynamics. Permafrost aggradation and degradation is ascribed to regional climatic, local autogenic and other factors. Particularly the very recent palsa development can be assessed in terms of climatic changes as inferred from meteorological data and surface hydrological changes related to construction of the railroad. The results indicate that cold years with limited snowfall as well as altered drainage patterns associated with infrastructure development may have contributed to the recent palsa formation.

Accumulation rate and stable oxygen isotope ratios of the ice cores from the North Greenland Traverse

We present for the first time all 12 d18O records obtained from ice cores drilled in the framework of the North Greenland Traverse (NGT) between 1993 and 1995 in northern Greenland. The cores cover an area of 680 km × 317 km, 10 % of the Greenland ice sheet. Depending on core length (100-175 m) and accumulation rate (90-200 kg/m**2/a) the single records reflect an isotope-temperature history over the last 500-1100 years. Lowest d18O mean values occur north of the summit and east of the main divide as a consequence of Greenland's topography. In general, ice cores drilled on the main ice divide show different results than those drilled east of the main ice divide that might be influenced by secondary regional moisture sources. A stack of all NGT records and the NGRIP record is presented with improved signal-to-noise ratio. Compared to single records, this stack represents the mean d18O signal for northern Greenland that is interpreted as proxy for temperature. Our northern Greenland d18O stack indicates distinctly enriched d18O values during medieval times, about AD 1420 ± 20 and from AD 1870 onwards. The period between AD 1420 and AD 1850 has depleted d18O values compared to the average for the entire millennium and represents the Little Ice Age. The d18O values of the 20th century are comparable to the medieval period but are lower than that about AD 1420.

Temperature measured in permafrost borehole at ICDP site 5011-3 in El'gygytgyn Crater, 2008-2011

This study focuses on the temperature field observed in boreholes drilled as part of interdisciplinary scientific campaign targeting the El'gygytgyn Crater Lake in NE Russia. Temperature data are available from two sites: the lake borehole 5011-1 located near the center of the lake reaching 400 m depth, and the land borehole 5011-3 at the rim of the lake, with a depth of 140 m. Constraints on permafrost depth and past climate changes are derived from numerical simulation of the thermal regime associated with the lake-related talik structure. The thermal properties of the subsurface needed for these simulations are based on laboratory measurements of representative cores from the quaternary sediments and the underlying impact-affected rock, complemented by further information from geophysical logs and data from published literature. The temperature observations in the lake borehole 5011-1 are dominated by thermal perturbations related to the drilling process, and thus only give reliable values for the lowermost value in the borehole. Undisturbed temperature data recorded over more than two years are available in the 140 m deep land-based borehole 5011-3. The analysis of these observations allows determination of not only the recent mean annual ground surface temperature, but also the ground surface temperature history, though with large uncertainties. Although the depth of this borehole is by far too insufficient for a complete reconstruction of past temperatures back to the Last Glacial Maximum, it still affects the thermal regime, and thus permafrost depth. This effect is constrained by numerical modeling: assuming that the lake borehole observations are hardly influenced by the past changes in surface air temperature, an estimate of steady-state conditions is possible, leading to a meaningful value of 14 ± 5 K for the post-glacial warming. The strong curvature of the temperature data in shallower depths around 60 m can be explained by a comparatively large amplitude of the Little Ice Age (up to 4 K), with low temperatures prevailing far into the 20th century. Other mechanisms, like varying porosity, may also have an influence on the temperature profile, however, our modeling studies imply a major contribution from recent climate changes.

Table 2. Lead and Ba concentrations and Pb isotopic compositions measured in Law Dome ice cores

Lead isotopic compositions and Pb and Ba concentrations have been measured in ice cores from Law Dome, East Antarctica, covering the past 6500 years. 'Natural' background concentrations of Pb (ab. 0.4 pg/g) and Ba (ab. 1.3 pg/g) are observed until 1884 AD, after which increased Pb concentrations and lowered 206Pb/207Pb ratios indicate the influence of anthropogenic Pb. The isotopic composition of 'natural' Pb varies within the range 206Pb/207Pb=1.20-1.25 and 208Pb/207Pb=2.46-2.50, with an average rock and soil dust Pb contribution of 8-12%. A major pollution event is observed at Law Dome between 1884 and 1908 AD, elevating the Pb concentration four-fold and changing 206Pb/207Pb ratios in the ice to ab. 1.12. Based on Pb isotopic systematics and Pb emission statistics, this is attributed to Pb mined at Broken Hill and smelted at Broken Hill and Port Pirie, Australia. Anthropogenic Pb inputs are at their greatest from 1900 to 1910 and from ab. 1960 to ab. 1980. During the 20th century, Ba concentrations are consistently higher than 'natural' levels and are attributed to increased dust production, suggesting the influence of climate change and/or changes in land coverage with vegetation.

Stable isotopic composition of Poritres lutea from New Caledonia

A 335 year stable isotope record from a New Caledonia coral (22°S, 166°E) helps fill a large gap in historical climate reconstructions. Although the long-term coral s18O-based sea surface temperature (SST) trend is one of warming, there are notable decadal fluctuations, especially in the early 18th and early 19th centuries. Mean annual SSTs between 1658 and 1900 are estimated to be ~0.3°C lower than the 20th century average, with interdecadal excursions of 0.5°-0.8°C. Time series analyses of the coral isotope record reveals significant concentrations of variance in the El Niño band; an inderdecadal spectral peak is present, but its robustness requires additional statistical evaluation. A secular but irregular decrease in coral d13C values begins in the mid-1800s and may reflect the anthropogenic perturbation of the carbon reservoir. These and other results indicate that the New Caledonia coral isotope record is a valuable source of information on southwest Pacific climate history.

Ice observations in the Southern Ocean between 1878 and 1928

During the period in question, large ice drifts transported incalculable numbers of icebergs, ice fields and ice floes from the Antarctica into the South Atlantic, confronting long-journeying sailing ships on the Cape Horn route with considerable danger. As is still the case today, the ice drifts generally tended in a northeasterly direction. Thus it can be assumed that the ice masses occuring near Cape Horn and in the South Atlantic originated in Graham Land and the South Shetland Islands, while those found in the Pacific will have come from Victoria Land. The masses drifting to Cape Horn, Isla de los Estados, the Falkland Islands and occasionally as far as the Tristan da Cunha Group are transported by the West Wind Drift and Falkland Current, diverted by the Brazil Current. The Bouvet and Agulhas Currents have little influence here. The great ice masses repeatedly reached points beyond the "outermost drift ice boundery" calculated in the course of the years, to continue on in the direction of the equator. The number of sailing ships which fell victim to the ice drifts while rounding Cape Horn can only be surmised; they simply disappeared without a trace in the expanses of the South Atlantic. Until the end of the 1900s the dangers presented by ice were less serious for westward-bound ships than for the "homeward-bounders" travelling from West to East. Following the turn of the century, however, the risk for "onwardbounders" increased significantly. Whether the ice drifts actually grew in might or whether the more frequent and more detailed reports led to this impression, could never be ascertained by the German Hydrographie Office. In the forty-one years between 1868 and 1908, ten light, ten medium and nine heavy ice years were counted, and only twelve years in which no reports of ice were submitted to the German Hydrographie Office. "One of the most terrible dangers threatening ships on their return from the Pacific Ocean," the pilot book for the Atlantic Ocean warns, "is the encounter with ice, to be expected south of the 50th parallel (approx.) in the Pacific and south of the 40th parallel (approx.) in the South Atlantic." Following the ice drift of 1854-55, thought to be the first ever recorded, the increasing numbers of sailing ships rounding Cape Horn were frequently confronted with drifts of varying sizes or with single icebergs. Then from 1892-94, a colossal ice drift crossed the path of the sailships in three stages. Several sailing ships collided with the icebergs and could be counted lucky if they survived with heavy damage to the bow and the fo regear. The reports on those which vanished for ever in the ice masses are hardly of investigative value. The English suffered particularly badly in the ice-plagued waters; their captains apparently sailed courses that led more freqently through drifts than did the sailing instructions of the German Hydrographic Office. Thus, among others, Capt. Jarvis' DUNTRUNE, also the STANMORE, ARTHURSTONE and LORD RANOCH as well as the French GALATHEE and CASHMERE all collided with icebergs. The crew of the AETHELBERTH panicked after a collision and took to their lifeboats. It was only after the ship detached itself from the iceberg it had rammed that the men returned to it and continued their journey. The TEMPLEMORE, on the other hand, had to be abandoned for good. Of the German sailing ships, the FLOTOW is to be mentioned here, and in the third phase of the drift the American SAN JOAQUIN lost a large proportion of its rigging. In the 20th century ice drifts continued to cross the courses of the Cape Horn ships. 1906 and 1908 were recorded as particularly heavy ice years. In 1908-09 both the FALKLANDBANK and the TOXTETH fell prey to ice, or so it was assumed during the subsequent Maritime Board proceedings. For the most part the German sailing ships were spared greater damages by sea. Their captains sent detailed ice reports to the German Hydrographic Office, which gratefully welcomed the information and partially incorporated it in the third and final edition of the "Pilot Book for the Atlantic Ocean." From the end of 1926 until the beginning of 1928, the last of the large sailing ships were once again confronted with "tremendous masses of icebergs and ice drifts." Reports of this period originated above all on the P-Liners PADUA, PAMIR, PASSAT, PEKING, PINNAS, PRIWALL and the ships of Gustav Erikson's fleet. The fate of the training sailship ADMIRAL KARPFANGER in connection with the ice in early 1938 was never clearly determined by the Maritime Board proceedings. Collision with an iceberg, however, is thought to be the most likely cause of accident. Today freight sailing ships no longer cross the oceans. The Cape Horn route is relatively insignificant for engine-powered ships and icebergs can be spotted in plenty of time by modern navigation technology ... The large ice drifts are no longer a menace, but only a marginal note in the final chapter of the history of transoceanic sailing.