http://www.clim-past-discuss.net/Climate of the Past Discussions Latest Articlesenhttp://www.clim-past-discuss.net/9/2557/2013/<b>Pre-LGM Northern Hemisphere paleo-ice sheet topography</b><br /><br />Climate of the Past Discussions, 9, 2557-2587, 2013<br /><br />Author(s): J. Kleman, J. Fastook, K. Ebert, J. Nilsson, and R. Caballero<br /><br />We here reconstruct the paleotopgraphy of Northern Hemisphere ice sheets during the glacial maxima of marine isotope stages (MIS) 5b and 4. We employ two approaches, geologically based reconstruction and numerical modeling, in mutually supportive roles to arrive at probable ice sheet extents and topographies for each of these two time slices. For a physically based 3-D calculation based on geologically derived 2-D constraints, we use the University of Maine Ice Sheet Model (UMISM) to calculate ice-sheet thickness and topography. The approach and ice-sheet modeling strategy is designed to provide robust data sets of sufficient resolution for atmospheric circulation experiments for these previously elusive time periods. Two tunable parameters, a temperature scaling function applied to a spliced Vostok-GRIP record, and spatial adjustment of climatic pole position, were employed iteratively to achieve a good fit to geological constraints where such were available. The model credibly reproduces the first-order pattern of size and location of geologically indicated ice sheets during marine isotope stages (MIS) 5b (86.2 kyr model age) and 4 (64 kyr model age). From the interglacial state of two north-south obstacles to atmospheric circulation (Rocky Mountains and Greenland), by MIS 5b combined Quebec-Central Arctic and Scandinavian–Barents/Kara ice sheets had effectively increased the number of such highland obstacles to four. This number remained constant through MIS 4, but at the last glacial maximum (LGM) dropped to three, through the merging of the Cordilleran and the proto-Laurentide Ice Sheet to a single continent-wide North American ice sheet.Fri, 17 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2517/2013/<b>High-resolution glacial and deglacial record of atmospheric methane by continuous-flow and laser spectrometer analysis along the NEEM ice core</b><br /><br />Climate of the Past Discussions, 9, 2517-2556, 2013<br /><br />Author(s): J. Chappellaz, C. Stowasser, T. Blunier, D. Baslev-Clausen, E.J. Brook, R. Dallmayr, X. Faïn, J.E. Lee, L.E. Mitchell, O. Pascual, D. Romanini, J. Rosen, and S. Schüpbach<br /><br />The Greenland NEEM (North Greenland Eemian Ice Drilling) operation in 2010 provided the first opportunity to combine trace-gas measurements by laser spectroscopic instruments and continuous-flow analysis along a freshly drilled ice core in a field based setting. We present the resulting atmospheric methane (CH₄) record covering the time period from 107.7 to 9.5 ka b2k (thousand years before 2000 AD). Companion discrete CH₄ measurements are required to transfer the laser spectroscopic data from a relative to an absolute scale. However, even on a relative scale, the high-resolution CH₄ dataset significantly improves our knowledge of past atmospheric methane concentration changes. New significant sub-millennial-scale features appear during interstadials and stadials, generally associated with similar changes in water isotopic ratios of the ice, a proxy for local temperature. In addition to the mid-point of Dansgaard/Oeschger (D/O) CH₄ transitions usually used for cross-dating, sharp definition of the start and end of these events brings precise depth markers (with ±20 cm uncertainty) for further cross-dating with other ice core or paleo records, e.g. speleothems. The method also provides an estimate of CH₄ rates of change. The onsets of D/O events in the methane signal show a more rapid rate of change than their endings. The rate of CH₄ increase associated with the onsets of D/O events progressively declines from 1.7 to 0.6 ppbv yr^&minus;1 in the course of Marine Isotope Stage 3. The largest observed rate of increase takes place at the onset of D/O event #21 and reaches 2.5 ppbv yr^&minus;1.Mon, 13 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2489/2013/<b>Biogeochemical properties and diagenetic changes during the past 3.6 Ma recorded by FTIR spectroscopy in the sediment record of Lake El'gygytgyn, Far East Russian Arctic</b><br /><br />Climate of the Past Discussions, 9, 2489-2515, 2013<br /><br />Author(s): C. Meyer-Jacob, H. Vogel, M. Melles, and P. Rosén<br /><br />A number of studies have shown that Fourier transform infrared spectroscopy (FTIRS) can be applied to quantitatively assess lacustrine sediment constituents. In this study, we developed calibration models based on FTIRS for the quantitative determination of biogenic silica (BSi; <i>n</i> = 420; gradient: 0.9–56.5%), total organic carbon (TOC; <i>n</i> = 309; gradient: 0.02–2.89%), and total inorganic carbon (TIC; <i>n</i> = 153; gradient: 0.01–1.46%) in a 318 m long sediment record with a basal age of 3.6 Ma from Lake El'gygytgyn, Far East Russian Arctic. The developed partial least squares regression (PLSR) models yield high cross-validated (CV) <i>R</i>²_CV = 0.85–0.91 and low root mean square error of cross-validation (RMSECV) (2.1–4.3% of the gradient for the different properties). The FTIRS-inferred concentrations of BSi, TOC, and TIC provide an initial insight into the climatic and environmental evolution at Lake El'gygytgyn throughout the late Pliocene and Quaternary showing a considerably high bioproductivity in the lake ecosystem between ~ 3.27–3.54 Ma during the early Pliocene warm period. Moreover, we found that the recorded FTIR spectra contain information on sample burial depth as a result of diagenetic changes (dehydration/dehydroxilation) of certain mineral phases. Despite the indicated post-depositional processes, the calibration models yield good statistical performances showing that general FTIRS models can be developed for several hundred meters long records extending several million years back in time. Our results highlight FTIRS to be a rapid, cost-effective alternative to conventional methods for quantification of biogeochemical properties.Mon, 13 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2471/2013/<b>A new Himalayan ice core CH₄ record: possible hints on the preindustrial latitudinal gradient</b><br /><br />Climate of the Past Discussions, 9, 2471-2487, 2013<br /><br />Author(s): S. Hou, J. Chappellaz, D. Raynaud, V. Masson-Delmotte, J. Jouzel, P. Bousquet, and D. Hauglustaine<br /><br />Two ice cores recovered from the Himalayan East Rongbuk (ER) Glacier on the northeast saddle of Mt. Qomolangma (Everest) (28&deg; 01' N, 86&deg; 58' E, 6518 m a.s.l.) give access to a tentative record of past Himalayan atmospheric mixing ratio of CH₄ spanning the past 1200 yr. The major part of the record is affected by artefacts probably due to in-situ production. After selecting what may represent the true atmospheric mixing ratio, an average of 749 &plusmn; 25 ppbv of CH₄ is estimated for the late preindustrial Holocene, which is ~ 36 &plusmn; 17 (~ 73 &plusmn; 18) ppbv higher than the atmospheric levels recorded respectively in Greenland and Antarctic ice cores. A comparison of these new data with model simulations of the CH₄ latitudinal gradient suggests either that the models do not get a correct balance between high and low latitude CH₄ sources, or that the filtered CH₄ profile from the ER cores remains infected by small artefacts.Mon, 13 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2423/2013/<b>Droughts in the Czech Lands, 1090&ndash;2012 AD</b><br /><br />Climate of the Past Discussions, 9, 2423-2470, 2013<br /><br />Author(s): R. Brázdil, P. Dobrovoln&yacute;, M. Trnka, O. Kotyza, L. &Rcaron;ezn&iacute;čková, H. Valášek, P. Zahradn&iacute;ček, and P. Štěpánek<br /><br />This paper addresses droughts in the Czech Lands in the 1090–2012 AD period, basing its findings on documentary evidence and instrumental records. Various documentary sources were employed for the selection of drought events, which were then interpreted at a monthly level. While the data on droughts before 1500 AD are scarce, the analysis concentrated mainly on droughts after this time. A dry year in 1501–1804 period (i.e. pre-instrumental times) was defined as a calendar year in the course of which dry patterns occurred on at least two consecutive months. Using this definition, 129 dry years were identified (an average of one drought per 2.4 yr). From the 16th to the 18th centuries these figures become 41, 36 and 49 yr, respectively, with the prevailing occurrence of dry months from April to September (73.7%). Drought indices – SPEI-1, Z-index and PDSI – calculated for the Czech Lands for April–September describe drought patterns between 1805 and 2012 (the instrumental period). <i>N</i> year recurrence intervals were calculated for each of the three indices. Using <i>N</i> &ge; 5 yr, SPEI-1 indicates 40 drought years, Z-index 39 yr and PDSI 47 yr. SPEI-1 and Z-index recorded 100 yr drought in 1834, 1842, 1868, 1947 and 2003 (50 yr drought in 1992). PDSI as an indicator of long-term drought disclosed two important drought periods: 1863–1874 and 2004–2012. The first period was related to a lack of precipitation, the other may be attributed to recent temperature increases without significant changes in precipitation. Droughts from the pre-instrumental and instrumental period were used to compile a long-term chronology for the Czech Lands. The number of years with drought has fluctuated between 26 in 1951–2000 and 16 in 1651–1700. Only nine drought years were recorded between 1641 and 1680, while between 1981 and 2012 the figure was 22 yr. A number of past severe droughts are described in detail: in 1540, 1590, 1616, 1718 and 1719. A discussion of the results centres around the uncertainty problem, the spatial variability of droughts, comparison with tree-ring reconstructions from southern Moravia, and the broader Central European context.Wed, 08 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2401/2013/<b>Eurasian Arctic climate over the past millennium as recorded in the Akademii Nauk ice core (Severnaya Zemlya)</b><br /><br />Climate of the Past Discussions, 9, 2401-2422, 2013<br /><br />Author(s): T. Opel, D. Fritzsche, and H. Meyer<br /><br />The chronology of the Akademii Nauk (AN) ice core from Severnaya Zemlya (SZ) has been expanded to the last 1100 yr. Here, we present the easternmost high-resolution ice-core climate-proxy records (&delta;¹⁸O and sodium) from the Arctic that provide new perspectives on past climate fluctuations in the Barents and Kara seas region. Multi-annual AN &delta;¹⁸O data as near-surface air-temperature proxy reveal major temperature changes over the last millennium, including the absolute minimum around 1800 and the exceptional warming to a double-peak maximum in the early 20th century. Neither a pronounced Medieval Climate Anomaly nor a Little Ice Age are detectable in the AN &delta;¹⁸O record. In contrast, there is evidence for several abrupt warming and cooling events such as in the 15th and 16th centuries. These abrupt changes are probably caused by shifts in the atmospheric circulation patterns and accompanied sea-ice feedbacks in the Barents and Kara seas region that highlight the role of the internal variability of the Arctic climate system.Wed, 08 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2357/2013/<b>Water-soluble organic carbon in snow and ice deposited at Alpine, Greenland, and Antarctic sites: a critical review of available data and their atmospheric relevance</b><br /><br />Climate of the Past Discussions, 9, 2357-2399, 2013<br /><br />Author(s): M. Legrand, S. Preunkert, B. Jourdain, J. Guilhermet, X. Fain, I. Alekhina, and J. R. Petit<br /><br />While it is now recognized that organic matter dominates the present-day atmospheric aerosol load over continents, its sources remain poorly known. The studies of organic species or organic fractions trapped in ice cores may help to overcome this lack of knowledge. Available data on the dissolved (or total) organic carbon (DOC or TOC) content of snow and ice often appear largely inconsistent and until now no critical review was conducted to understand the causes of these inconsistencies. To draw a more consistent picture of the organic carbon amount present in solid precipitation that accumulates on cold glaciers, we here review available data and, when needed, complete the data set with analyses of selected samples. The different data sets are then discussed by considering the age (modern versus pre-industrial, Holocene versus last glacial maximum) and type (surface snow, firn, or ice) of investigated samples, the deployed method (DOC, TOC) and the applied contamination control. Finally, the OC levels of Antarctic, Greenland and Alpine ice cores are compared and discussed with respect to natural (biomass burning, vegetation emissions) and anthropogenic source (fossil fuel combustion) contributions to atmospheric OC aerosol.Tue, 07 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2309/2013/<b>Holocene climate variability in the Winter Rainfall Zone of South Africa</b><br /><br />Climate of the Past Discussions, 9, 2309-2356, 2013<br /><br />Author(s): S. Weldeab, J.-B. W. Stuut, R. R. Schneider, and W. Siebel<br /><br />We established a multi-proxy time series comprising analyses of major elements in bulk sediments, Sr and Nd isotopes and grain size of terrigenous fraction, and &delta;¹⁸O and &delta;¹³C in tests of <i>Neogloboquadrina pachyderma</i> (sinistral) from a marine sediment sequence recovered off the Orange River. The records reveal coherent patterns of variability that reflect changes in wind strength, precipitation over the river catchments, and upwelling of cold and nutrient-rich coastal waters off western South Africa. The wettest episode of the Holocene in the Winter Rainfall Zone (WRZ) of South Africa occurred during the "Little Ice Age" (700–100 yr BP). Wet phases were accompanied by strengthened coastal water upwellings, a decrease of Agulhas water leakage into the southern Atlantic, and a reduced dust incursion over Antarctica. A continuous aridification trend in the WRZ and a weakening of the southern Benguela Upwelling System (BUS) between 9000 and 5500 yr BP parallel with increase of dust deposition over Antarctica and an enhanced leakage of warm Agulhas water into the southeastern Atlantic. The temporal relationship between precipitation changes in the WRZ, the thermal state of the coastal surface water, and leakage of warm water in southern Atlantic, and variation of dust incursion over Antarctica suggests a causal link that most likely was related to latitudinal shifts of the Southern Hemisphere westerlies. Our results of the mid-Holocene time interval may serve as an analogue to a possible long-term consequence of the current and future southward shift of the westerlies that may result in a decline of rainfall over southwest Africa and a weakened upwelling with implication for phytoplankton productivity and fish stocks. Furthermore, warming of the coastal surface water as a result of warm Agulhas water incursion into the southern BUS may affect coastal fog formation that is critical as moisture source for the endemic flora of the Namaqualand.Tue, 07 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2277/2013/<b>Late Holocene summer temperatures in the central Andes reconstructed from the sediments of high-elevation Laguna Chepical, Chile (32° S)</b><br /><br />Climate of the Past Discussions, 9, 2277-2308, 2013<br /><br />Author(s): R. de Jong, L. von Gunten, A. Maldonado, and M. Grosjean<br /><br />High-resolution reconstructions of climate variability that cover the past millennia are necessary to improve the understanding of natural and anthropogenic climate change across the globe. Although numerous records are available for the mid- and high-latitudes of the Northern Hemisphere, global assessments are still compromised by the scarcity of data from the Southern Hemisphere. This is particularly the case for the tropical and subtropical areas. In addition, high elevation sites in the South American Andes may provide insight into the vertical structure of climate change in the mid-troposphere. This study presents a 3000 yr long austral summer (November to February) temperature reconstruction derived from the ²¹⁰Pb and ¹⁴C dated organic sediments of Laguna Chepical (32°16' S/70°30' W, 3050 m a.s.l.), a high-elevation glacial lake in the subtropical Andes of central Chile. Scanning reflectance spectroscopy in the visible light range provided the spectral index <i>R</i>₅₇₀/<i>R</i>₆₃₀, which reflects the clay mineral content in lake sediments. For the calibration period (AD 1901–2006), the <i>R</i>₅₇₀/<i>R</i>₆₃₀ data were regressed against monthly meteorological reanalysis data, showing that this proxy was strongly and significantly correlated with mean summer (NDJF) temperatures (<i>R</i>_3yr = &minus;0.63, <i>p</i>_adj = 0.01). This calibration model was used to make a quantitative temperature reconstruction back to 1000 BC. <br><br> The reconstruction (with a model error RMSEP_boot of 0.33 °C) shows that the warmest decades of the past 3000 yr occurred during the calibration period. The 19th century (end of the Little Ice Age (LIA)) was cool. The prominent warmth reconstructed for the 18th century, which was also observed in other records from this area, seems systematic for subtropical and southern South America but remains difficult to explain. Except for this warm period, the LIA was generally characterized by cool summers. Back to AD 1400, the results from this study compare remarkably well to low altitude records from the Chilean Central Valley and Southern South America. However, the reconstruction from Laguna Chepical does not show a warm Medieval Climate Anomaly during the 12–13th century, which is consistent with records from tropical South America. The Chepical record also indicates substantial cooling prior to 800 BC. This coincides with well-known regional as well as global glacier advances which have been attributed to a grand solar minimum. This study thus provides insight into the climatic drivers and temperature patterns in a region for which currently very few data are available. It also shows that since ca AD 1400, long term temperature patterns were generally similar at low and high altitudes in central Chile.Mon, 06 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2235/2013/<b>Major dust events in Europe during marine isotope stage 5 (130&ndash;74 ka): a climatic interpretation of the "markers"</b><br /><br />Climate of the Past Discussions, 9, 2235-2276, 2013<br /><br />Author(s): D.-D. Rousseau, M. Ghil, G. Kukla, A. Sima, P. Antoine, M. Fuchs, C. Hatté, F. Lagroix, M. Debret, and O. Moine<br /><br />At present, major dust storms are occurring at mid-latitudes in the Middle East and Asia, as well as at low latitudes in northern Africa and in Australia. Western Europe, though, does not experience such dramatic climate events, except for some African dust reaching it from the Sahara. This modern situation is of particular interest, in the context of future climate projections, since the present interglacial is usually interpreted, in this context, as an analog of the warm Eemian interval. European terrestrial records show, however, major dust events during the penultimate interglacial and early glacial. These events are easily observed in loess records by their whitish-color deposits, which lie above and below dark chernozem paleosols in Central European records of Marine Isotope Stage (MIS) 5 age. <br><br> We describe here the base of the Dolni Vestonice (DV) loess sequence, Czech Republic, as the reference of such records. The dust is deposited during intervals that are characterized by poor vegetation – manifested by high &delta;¹³C values and low magnetic susceptibility – while the fine sand and clay in the deposits shows grain sizes that are clearly different from the overlying pleniglacial loess deposits. Some of these dust events have been previously described as "Markers" or Marker Silts (MS) by one of us (G. Kukla), and are dated at about 111–109 and 93–92 ka, with a third and last one slightly visible at about 75–73 ka. Other events correspond to the loess material of Kukla's cycles, and are described as eolian silts (ES); they are observed in the same DV sequence and are dated at about 106–105, 88–86, and 78.5–77 ka. <br><br> The fine eolian deposits mentioned above, MS as well as ES, correspond to short events that lasted about 2 ka; they are synchronous with re-advances of the polar front over the North Atlantic, as observed in marine sediment cores. These deposits also correlate with important changes observed in European vegetation. Some ES and MS events appear to be coeval with significant dust peaks recorded in the Greenland ice cores, while others are not. This decoupling between the European eolian and Greenland dust depositions is of considerable interest, as it differs from the fully glacial situation, in which the Eurasian loess sedimentation mimics the Greenland dust record. Previous field observations supported an interpretation of MS events as caused by continental dust storms. We show here, by a comparison with speleothems of the same age found in the northern Alps, that different atmospheric-circulation modes seem to be responsible for the two categories of dust events, MS vs. ES.Thu, 02 May 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2217/2013/<b>Re-evaluation of the age model for North Atlantic Ocean Site 982 &ndash; arguments for a return to the original chronology</b><br /><br />Climate of the Past Discussions, 9, 2217-2233, 2013<br /><br />Author(s): K. T. Lawrence, I. Bailey, and M. E. Raymo<br /><br />Recently, the veracity of the published chronology for the Pliocene section of North Atlantic Ocean Drilling Program Site 982 was called into question. Here, we examine the robustness of the original age model as well as the proposed age model revision. The proposed revision is predicated on an apparent mis-identification of the depth to the Gauss-Matuyama (G/M) polarity chronozone reversal boundary (2.581 Ma) based on preliminary shipboard paleomagnetic data and offers a new chronology which includes a hiatus between ~ 3.2 and 3 Ma. However, an even more accurate shore-based, u-channel-derived polarity chronozone stratigraphy for the past ~ 2.7 Ma supports the shipboard composite stratigraphy and demonstrates that the original estimate of the depth of the G/M reversal in the Site 982 record is correct. Thus, the main justification forwarded to support the revised chronology is not valid. We demonstrate that the proposed revision results in a pronounced anomaly in sedimentation rates proximal to the proposed hiatus, erroneous assignment of marine-isotope stages in the Site 982 Pliocene benthic stable oxygen isotope stratigraphy, and a markedly worse correlation of proxy records between this site and other regional paleoclimate data. We conclude that the original chronology for Site 982 is a far more accurate age-model than that which arises from the published revision. We strongly recommend the use of the original chronology for all future work at Site 982.Mon, 29 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2183/2013/<b>Interdependence of the Northern Hemisphere ice-sheets build-up during the last glaciation: the role of atmospheric circulation</b><br /><br />Climate of the Past Discussions, 9, 2183-2216, 2013<br /><br />Author(s): P. Beghin, S. Charbit, C. Dumas, M. Kageyama, D. M. Roche, and C. Ritz<br /><br />The development of large continental-scale ice sheets over Canada and Northern Europe during the last glacial cycle likely modified the track of stationary waves and influenced the location of growing ice sheets through changes in accumulation and temperature patterns. Although they are often mentioned in the literature, these feedback mechanisms are poorly constrained and have never been studied throughout an entire glacial-interglacial cycle. Using the climate model of intermediate complexity CLIMBER-2 coupled with the 3-D ice-sheet model GRISLI, we investigate the impact of stationary waves on the construction of past Northern Hemisphere ice sheets during the past glaciation. The stationary waves are not explicitly computed in the model but their effect on sea-level pressure is parameterized. Several parameterizations have been tested allowing to study separately the effect of surface temperature (thermal forcing) and topography (orographic forcing) on sea-level pressure, and therefore on atmospheric circulation and ice-sheet surface mass balance. We show that the response of ice sheets to thermal and/or orographic forcings is rather different. At the beginning of the glaciation, the orographic effect favors the growth of the Laurentide ice sheet, whereas Fennoscandia appears rather sensitive to the thermal effect. Using the ablation parameterization as a trigger to artificially modify the size of one ice sheet, the remote influence of one ice sheet on the other is also studied as a function of the stationary wave parameterizations. The sensitivity of remote ice sheets is shown to be highly sensitive to the choice of these parameterizations with a larger response when orographic effect is accounted for. Results presented in this study suggest that the various spatial distributions of ice sheets could be partly be explained by the feedbacks mechanisms occurring between ice sheets and atmospheric circulation.Thu, 18 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2153/2013/<b>Accumulation reconstruction and water isotope analysis for 1735&ndash;1997 of an ice core from the Ushkovsky volcano, Kamchatka, and their relationships to North Pacific climate records</b><br /><br />Climate of the Past Discussions, 9, 2153-2181, 2013<br /><br />Author(s): T. Sato, T. Shiraiwa, R. Greve, H. Seddik, E. Edelmann, and T. Zwinger<br /><br />To investigate past climate change in the Northwest Pacific region, an ice core was retrieved in June 1998 from the Gorshkov crater glacier at the top of the Ushkovsky volcano, in central Kamchatka. Hydrogen isotope (&delta;D) analysis and past accumulation reconstructions were conducted to a depth of 140.7 m, dated to 1735. Two accumulation reconstruction methods were applied with the Salamatin and the Elmer/Ice ice flow models. Reconstructed accumulation rates and &delta;D were significantly correlated with North Pacific surface temperature. This, and a significant correlation of &delta;D with the North Pacific Gyre Oscillation (NPGO) index implies that NPGO data is contained in this record. Wavelet analysis shows that the ice core records have significant multi-decadal power spectra up to the late 19th century. The multi-decadal periods of reconstructed accumulation rates change at around 1850 in the same way as do Northeast Pacific ice core and tree ring records. The loss of multi-decadal scale power spectra of &delta;D and the 6&permil; increase in its average value occurred around 1880. Thus the core record confirms that the periodicity of precipitation for the entire North Pacific changed between the end of the Little Ice Age through the present due to changes in conditions in the North Pacific Ocean.Wed, 17 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2115/2013/<b>The role of East-Tethys seaway closure in the middle Miocene climatic transition (ca. 14 Ma)</b><br /><br />Climate of the Past Discussions, 9, 2115-2152, 2013<br /><br />Author(s): N. Hamon, P. Sepulchre, V. Lefebvre, and G. Ramstein<br /><br />The middle Miocene climatic transition (MMCT, approximately 14 Ma) is a key period in Cenozoic cooling and cryospheric expansion. Despite it is well documented in isotopic record, the causes of the MMCT are still a matter of debate. Among various hypotheses, some authors suggested that it was linked with the final closure of the East-Tethys seaway and subsequent oceanic circulation reorganisation. The aim of the present study is to quantify the impact of varying East-Tethys seaway depths on middle Miocene ocean and climate, in order to better understand its role in the MMCT. <br><br> We present four sensitivity experiments with a fully coupled ocean-atmosphere generalized circulation model. Our results indicate the presence of a warm and salty water source in the northern Indian Ocean when the East-Tethys is deep-open (4000 or 1000 m), which corresponds to the Tethyan Indian Saline Water (TISW) described on the basis of isotopic studies. This water source is absent in the experiments with shallow (250 m) and closed East-Tethys, inducing strong changes in the latitudinal density gradient and ultimately the reinforcement of the Antarctic Circumpolar Current (ACC). Moreover, when the East-Tethys seaway is shallow or closed, there is a westward water flow in the Gibraltar Strait that strengthens the Atlantic meridional overturning circulation (AMOC) compared to the experiments with deep-open East-Tethys. Our results therefore suggest that the shoaling and final closure of the East-Tethys seaway played a major role in the oceanic circulation reorganisation during the middle Miocene. <br><br> The results presented here provide new constraints on the timing of the East-Tethys seaway closure, and particularly indicate that, prior to 14 Ma, a deep-open East-Tethys should have allow the formation of TISW. Moreover, whereas the final closure of this seaway likely played a major role in the MMCT, we suggest that it was not the only driver of the global cooling and Antarctica ice sheet growth. Here, we propose that the initiation of the MMCT may have been an atmospheric <i>p</i>CO₂ drawdown and that the oceanic Changes due to the East-Tethys seaway closure amplified the response of global climate and East-Antarctic Ice Sheet.Mon, 15 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2095/2013/<b>Paleo Agulhas rings enter the subtropical gyre during the penultimate deglaciation</b><br /><br />Climate of the Past Discussions, 9, 2095-2114, 2013<br /><br />Author(s): P. Scussolini and E. van Sebille<br /><br />A maximum in the strength of Agulhas Leakage has been registered at the interface between Indian and South Atlantic oceans during glacial Termination II (T II), presumably transporting the salt and heat necessary to maintain the Atlantic Meridional Overturning Circulation (AMOC) at rates similar to the present day. However, it was never shown whether these were effectively incorporated in the South Atlantic gyre, or whether they retroflected into the Indian and/or Southern Oceans. To solve this question, we investigate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic basin than previously studied. Analysis of a 20 yr dataset from a global ocean circulation model allows us to relate density perturbations, at the depth of the thermocline, to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of &delta;¹⁸O variability of <i>Globorotalia truncatulinoides</i> single specimens, revealing high levels of pycnocline depth variability at the site, suggesting enhanced numbers of Agulhas rings moving into the South Atlantic gyre around and before T II. Our record closely follows the published quantifications of Agulhas Leakage from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the South Atlantic circulation. This provides crucial support to the view of a prominent role of the Agulhas Leakage in the shift from a glacial to an interglacial mode of AMOC.Thu, 11 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2059/2013/<b>The last 7 millennia of vegetation and climate changes at Lago di Pergusa (central Sicily, Italy)</b><br /><br />Climate of the Past Discussions, 9, 2059-2094, 2013<br /><br />Author(s): L. Sadori, E. Ortu, O. Peyron, G. Zanchetta, B. Vannière, M. Desmet, and M. Magny<br /><br />The aim of this study is to investigate climate changes and human activities under the lens of palynology. Based on a new high-resolution pollen sequence (PG2) from Lago di Pergusa (667 m a.s.l., central Sicily, Italy) covering the last 6700 yr, we propose a reconstruction of climate and landscape changes over the recent past in central Sicily. Compared to former studies from Lago di Pergusa (Sadori and Narcisi, 2001), this work provides a reconstruction of the evolution of vegetation and climate over the last millennia in central Sicily, indeed completing previous results with new data which is particularly detailed on the last 3000 yr. <br><br> Joint actions of increasing dryness, climate oscillations, and human impact shaped the landscape of this privileged site. Lago di Pergusa, in fact, besides being the main inland lake of Sicily, is very sensitive to climate change and its territory was inhabited and exploited continuously since the prehistory. The lake sediments turned out to be a good observatory for the natural phenomena occurred in the last thousands of years. <br><br> Results of the pollen-based study are integrated with changes in magnetic susceptibility and a tephra layer characterization. The tephra layer was shown to be related to the Sicanians' event, radiocarbon dated at 3055 ± 75 yr BP (Sadori and Narcisi, 2001). <br><br> We performed palaeoclimate reconstructions by MAT and WA-PLS. Palaeoclimate reconstructions based on the core show important climate fluctuations throughout the Holocene. Climate reconstruction points out four phases of cooling and enhanced wetness in the last three millennia (2600–2000, 1650–1100, 850–550, 400–200 cal BP). This appears to be the evidence of local responses to global climate oscillations during the recent past.Mon, 08 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/2015/2013/<b>A high resolution record of atmospheric carbon dioxide and its stable carbon isotopic composition from the penultimate glacial maximum to the glacial inception</b><br /><br />Climate of the Past Discussions, 9, 2015-2057, 2013<br /><br />Author(s): R. Schneider, J. Schmitt, P. Köhler, F. Joos, and H. Fischer<br /><br />The reconstruction of the stable carbon isotope evolution in atmospheric CO₂ (&delta;¹³C_atm), as archived in Antarctic ice cores, bears the potential to disentangle the contributions of the different carbon cycle fluxes causing past CO₂ variations. Here we present a highly resolved record of &delta;¹³C_atm before, during and after the Marine Isotope Stage 5.5 (155 000 to 105 000 yr BP). The record was derived with a well established sublimation method using ice from the EPICA Dome C (EDC) and the Talos Dome ice cores in East Antarctica. We find an 0.4&permil; offset between the mean &delta;¹³C_atm level in the Penultimate (~140 000 yr BP) and Last Glacial Maximum (~22 000 yr BP), which can be explained by either (i) changes in the isotopic composition or (ii) intensity of the carbon input fluxes to the combined ocean/atmosphere carbon reservoir or (iii) by long-term peat buildup. Our isotopic data suggest that the carbon cycle evolution along Termination II and the subsequent interglacial was controlled by essentially the same processes as during the last 24 000 yr, but with different phasing and magnitudes. Furthermore, a 5000 yr lag in the CO₂ decline relative to EDC temperatures is confirmed during the glacial inception at the end of MIS 5.5 (120 000 yr BP). Based on our isotopic data this lag can be explained by terrestrial carbon release and carbonate compensation.Mon, 08 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/1969/2013/<b>Holocene vegetation and climate changes in central Mediterranean inferred from a high-resolution marine pollen record (Adriatic Sea)</b><br /><br />Climate of the Past Discussions, 9, 1969-2014, 2013<br /><br />Author(s): N. Combourieu-Nebout, O. Peyron, V. Bout-Roumazeilles, S. Goring, I. Dormoy, S. Joannin, L. Sadori, G. Siani, and M. Magny<br /><br />To understand the effects of future climate change on the ecology of the central Mediterranean we can look to the impacts of long-term, millennial to centennial-scale climatic variability on vegetation in the basin. Pollen data from the Adriatic Marine core MD 90-917 allows us to reconstruct vegetation and regional climate changes over the south central Mediterranean during the Holocene. Clay mineral ratios from the same core reflect the relative contributions of riverine (illite and smectite) and eolian (kaolinite) contributions to the site, and thus act as an additional proxy with which to test precipitation changes in the Holocene. <br><br> Vegetation reconstruction shows vegetation responses to the late-Glacial Preboreal oscillation, most likely driven by changes in seasonal precipitation. Pollen-inferred temperature declines during the early-mid Holocene, but increases during the mid-late Holocene, similar to southern-western Mediterranean climatic patterns during the Holocene. Several short climatic events appear in the record, indicating the sensitivity of vegetation in the region to millennial-scale variability. <br><br> Reconstructed summer precipitation shows a regional maximum between 8000 and 7000 cal yr BP similar to the general pattern across southern Europe. Two important shifts in vegetation occur at 7700 and between 7500 and 7000 yr. These vegetation shifts are linked to changes in seasonal precipitation and are correlated to increased river inputs respectively from the north (7700 event) and from the central Adriatic borderlands (7500–7000 event). These results reinforce the strengths of multi-proxy analysis and provide a deeper understanding of the role of precipitation and particularly the seasonality of precipitation in mediating vegetation change in the central Mediterranean during the Holocene.Fri, 05 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/1901/2013/<b>North–south palaeohydrological contrasts in the central Mediterranean during the Holocene: tentative synthesis and working hypotheses</b><br /><br />Climate of the Past Discussions, 9, 1901-1967, 2013<br /><br />Author(s): M. Magny, N. Combourieu Nebout, J. L. de Beaulieu, V. Bout-Roumazeilles, D. Colombaroli, S. Desprat, A. Francke, S. Joannin, O. Peyron, M. Revel, L. Sadori, G. Siani, M. A. Sicre, S. Samartin, A. Simonneau, W. Tinner, B. Vannière, B. Wagner, G. Zanchetta, F. Anselmetti, E. Brugiapaglia, E. Chapron, M. Debret, M. Desmet, J. Didier, L. Essallami, D. Galop, A. Gilli, J. N. Haas, N. Kallel, L. Millet, A. Stock, J. L. Turon, and S. Wirth<br /><br />On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north–south transect, data collected in the Central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and temporal coherency. Contrasting patterns of palaeohydrological changes have been evidenced in the Central Mediterranean: south (north) of around 40&deg; N of latitude, the middle part of the Holocene was characterised by lake-level maxima (minima), during an interval dated to ca. 10 300–4500 cal BP to the south and 9000–4500 cal BP to the north. Available data suggest that these contrasting palaeohydrological patterns operated throughout the Holocene, both on millennial and centennial scales. Regarding precipitation seasonality, maximum humidity in the Central Mediterranean during the middle part of the Holocene was characterised by humid winters and dry summers north of ca. 40&deg; N, and humid winters and summers south of ca. 40&deg; N. This may explain an apparent conflict between palaeoclimatic records depending on the proxies used for reconstruction as well as the synchronous expansion of tree species taxa with contrasting climatic requirements. In addition, south of ca. 40&deg; N, the first millennium of the Holocene was characterised by very dry climatic conditions not only in the Eastern, but also in the Central and the Western Mediterranean zones as reflected by low lake levels and delayed reforestation. These results suggest that, in addition to the influence of the Nile discharge reinforced by the African monsoon, the deposition of Sapropel 1 has been favoured (1) by an increase in winter precipitation in the northern Mediterranean borderlands, and (2) by an increase in winter and summer precipitation in the southern Mediterranean area. The climate reversal following the Holocene climate optimum appears to have been punctuated by two major climate changes around 7500 and 4500 cal BP. <br><br> In the Central Mediterranean, the Holocene palaeohydrological changes developed in response to a combination of orbital, ice-sheet and solar forcing factors. The maximum humidity interval in the south-central Mediterranean started at ca. 10 300 cal BP, in correlation with the decline (1) of the possible blocking effects of the North Atlantic anticyclone linked to maximum insolation, and/or (2) of the influence of the remnant ice sheets and fresh water forcing in the North Atlantic Ocean. In the north-central Mediterranean, the lake-level minimum interval began only around 9000 cal BP when the Fennoscandian ice-sheet disappeared and a prevailing positive NAO-type circulation developed in the North Atlantic area. The major palaeohydrological oscillation around 4500–4000 cal BP may be a non-linear response to the gradual decrease, with additional key seasonal and interhemispherical changes, in insolation. On a centennial scale, the successive climatic events which punctuated the entire Holocene in the central Mediterranean coincided with cooling events associated with deglacial outbursts in the North Atlantic area and decreases in solar activity during the interval 11 700–7000 cal BP, and to a possible combination of NAO-type circulation and solar forcing since ca. 7000 cal BP onwards. Thus, regarding the centennial-scale climatic oscillations, the Mediterranean Basin appears to have been strongly linked to the North Atlantic area and affected by solar activity over the entire Holocene. <br><br> In addition to model experiments, a better understanding of forcing factors and past atmospheric circulation patterns behind the Holocene palaeohydrological changes in the Mediterranean area will require further investigation to establish additional high-resolution and well-dated records in selected locations around the Mediterranean Basin and in adjacent regions. Special attention should be paid to greater precision in the reconstruction, on millennial and centennial time scales, of changes in the latitudinal location of the limit between the northern and southern palaeohydrological Mediterranean sectors, depending on (1) the intensity and/or characteristics of climatic periods/oscillations (e.g. Holocene thermal maximum versus Neoglacial, as well as, for instance, the 8.2 ka event versus the 4 ka event or the Little Ice Age), and (2) on varying geographical conditions from the western to the eastern Mediterranean areas (longitudinal gradients).Fri, 05 Apr 2013 00:00:00 +0200http://www.clim-past-discuss.net/9/1869/2013/<b>Southern Hemisphere orbital forcing and its effects on CO₂ and tropical Pacific climate</b><br /><br />Climate of the Past Discussions, 9, 1869-1900, 2013<br /><br />Author(s): K. Tachikawa, A. Timmermann, L. Vidal, C. Sonzogni, and O. E. Timm<br /><br />The western Pacific warm pool (WPWP) is an important heat source for the atmospheric circulation and influences climate conditions worldwide. Understanding its sensitivity to past radiative perturbations may help better contextualize the magnitudes and patterns of current and projected tropical climate change. Here we present a new Mg/Ca-based sea surface temperature (SST) reconstruction over the past 400 kyr from the Bismarck Sea, off Papua New Guinea, along with results from a transient earth system model simulation. Our results document the primary influence of CO₂ forcing on glacial/interglacial WPWP SSTs and secondary effects due to changes in wind-driven tropical boundary currents. In addition to the SST, deep ocean temperature reconstructions from this core are linked with Southern Ocean temperature and sea-ice variations on timescales of ~ 23 kyr. It is proposed that Southern Hemisphere insolation changes serve as pacemaker for sea-ice variations in the Southern Ocean, which in turn modulate windstress curl-driven upwelling of carbon-rich waters, hence controlling atmospheric CO₂ and tropical WPWP temperatures.Fri, 05 Apr 2013 00:00:00 +0200