Holocene Critical Zone dynamics in an Alpine catchment inferred from a speleothem multiproxy record : disentangling climate and human influences

Disentangling the effects of climate and human impact on the long-term evolution of the Earth CriticalZone is crucial to understand the array of its potential responses to the ongoing Global Change. This task requires natural archives from which local information about soil and vegetation can be linked directly to climate parameters. Here we present a high-resolution, well-dated, speleothem multiproxy record from the SW Italian Alps, spanning the last ~10,000 years of the present interglacial (Holocene). We correlate magnetic properties and the carbon stable isotope ratio to soil stability and pedogenesis, whereas the oxygen isotope composition is interpreted as primarily related to precipitation amount, modulated at different timescales by changes in precipitation source and seasonality. During the 9.7-2.8 ka period, when anthropic pressure over the catchment was scarce, intervals of enhanced soil erosion are related to climate-driven vegetation contractions and occurred during drier periods. Immediately following the onset of the Iron Age (ca. 2.8 ka), by contrast, periods of enhanced soil erosion coincided with a wetter climate. We propose that the observed changes in the soil response to climate forcing were related to early anthropogenic manipulations of Earth\u2019s surface, which made the ECZ more sensitive to climate oscillations

Stalagmite evidence for the onset of the Last Interglacial in southern Europe at 129±1 ka

Multi-proxy data from an Italian stalagmite constrain the commencement of full Last Interglacial conditions in southern Europe at 129 +/- 1 ka, consistent with absolutely dated records currently available from both hemispheres. The post-glacial transition towards warmer and wetter conditions commenced at 134 +/- 2 ka. Oxygen isotope evidence suggests this was interrupted briefly at 130 +/- 2 ka, an event probably related to the 'Termination II pause' associated with Heinrich Event 11. For most of the stalagmite, the pattern of delta(18)O variation mimics the air temperature record from the Vostok ice core, especially through marine isotope stage 5. There is no obvious evidence for substantial 'early interglacial' warming

Fossil dripwater in stalagmites reveals Holocene temperature and rainfall variation in Amazonia

Most proxy records used for reconstruction of Holocene climate of Amazonia are unable to quantitatively distinguish between the effect of temperature and rainfall amounts. We present a new isotope technique applied to a ∼ 13,500 yr stalagmite archive from Peruvian Amazonia. By analysing the coupled isotope composition of fossil dripwater trapped in stalagmite fluid inclusions, and that of the calcite hosting the fluid inclusions, we were able to calculate independent paleotemperatures and rainfall amounts. This stalagmite record shows that Holocene climate variation was controlled by orbitally-forced Southward migration of the Inter Tropical Convergence Zone. While temperature remained constant, isotope variation of rainwater, reflected in fluid inclusion water

Stratigraphic evidence for a “pluvial phase” between ca. 8200-7100 ka from Renella Cave (Central Italy)

A stratigraphic and chronological study of the upper level of Renella Cave (Apuan Alps, Central Italy) reveals that two episodes of flowstone accumulation bracket a thick clastic layer deposited between ca 8.2 and 7.1 ka. This layer, which represents a period of enhanced cave flooding, is substantially in phase with an interval of depleted oxygen isotope values previously recorded in a stalagmite from nearby Corchia Cave, interpreted to have resulted from an increase in local precipitation. These data confirm that during this period of time the region experienced relatively wetter conditions, including an increase in high-magnitude events capable of invading the higher passages of Renella Cave. The timing of the clastic phase occurred when the Eastern Mediterranean experienced deposition of sapropel layer S1, which is thought to reflect the stagnation of sea water produced largely by enhanced flood activity along the Nile in response to increased monsoon intensity in northern equatorial Africa. Recent estimates suggest that S1 may have lasted from ca 10.8 to ca 6.1 ka cal BP. Combined evidence from Renella and Corchia Cave indicates that the period corresponding to the wettest phase in the Apuan Alps was much shorter than this, and suggests that there is no straightforward connection between increased advection of water vapour from the Atlantic between 8.2 and 7.1 ka, as recorded in the Corchia and Renella records, and monsoon-driven enhancement of Nile discharge and S1 deposition in the eastern Mediterranea

Coeval dry events in the central and eastern Mediterranean basin at 5.2 and 5.6ka recorded in Corchia (Italy) and Soreq caves (Israel) speleothems

Soreq (Israel) and Corchia (central Italy) Caves are located 2500km far apart along the Mediterranean winter-storm track and are ideally suited for investigating past variations of winter rainfall in the Mediterranean region. Analyses of speleothem δ18O records from both caves for the period between ca. 7 to 4 ka BP show some striking similarities for the ca. 6 and 4ka interval, but lack agreement between ca. 7 to 6kaBP. Two prominent isotopic excursions, argued to reflect relatively drier conditions, are centred at ca. 5.6 and ca. 5.2 ka. The 5.2 ka event lasts less than a century, whereas the 5.6ka event extends from ca. 5.7 to 5.4 ka. A period of progressive drying is also apparent from ca. 5 to 4ka. Another prominent event, reflecting wetter conditions, is recorded in both records at ca. 5.8ka and seems to last several decades. The 5.6 and 5.2ka events occurred within a period of higher deposition of haematite-stained grains in cores of the sub-polar North Atlantic, and correlation with the wind strength proxy record from Hólmsá loess profile in Iceland suggests that rainfall reduction was related to a reduced vapour advection from Atlantic towards the Mediterranean connected to northward shift in the Westerlies. A comparison with Alpine records, including the Spannagel Cave isotope record, suggests that dry events recorded at Soreq and Corchia caves may correspond to wetter (lake high stands) and cooler (glacier expansion) conditions in the Alpine region, indicating complex regional climate re-organizatio

Speleothem U/Th age constraints for the Last Glacial conditions in the Apuan Alps, northwestern Italy

International audienceDuring the Quaternary several glaciations occurred in the mountain regions around the Mediterranean and, in recent years, new ages have better constrained their timing. However, this is not the case for the Apuan Alps, a high-rainfall mountain chain adjacent to the Mediterranean Sea. Here, in spite of the widespread evidence for glaciers, the complete lack of geochronological information hinders our understanding of glaciation history. In this paper, we utilize speleothem ages to better constrain the timing of these glacial features. We re-examine 293 uranium‑thorium ages from 19 speleothems collected in five caves at different elevations. After a period of very low growth between 160 and 132 ka, the analysed speleothems grew almost continuously to ~75 ka, this period was followed by intermittent growth with lower deposition rate and presence of hiatuses until ~12.5/12 ka. This is consistent with an ice coverage persisting over the Apuan Alps, inhibiting or interrupting the growth of speleothems via the limited availability of groundwater and the scarcity/absence of soils. This interval is much greater than the time interval that has previously been attributed to the existence of glaciers on the Apuan Alps, which has been assumed to be restricted to Marine Isotope Stage (MIS) 2. Instead, ice cover probably also appeared in the Apuan Alps during MIS 4. The phase of restarting of growth, which may implies the definitive or substantial glacier melts seem to predate the Holocene

Precise microsampling of poorly laminated speleothems for U-series dating

PosterInternational audienceSpeleothems are recognised as important palaeoclimate archives partly because of the ability to date them accurately and precisely by uranium-series (U-series) methods. The practicalities of sampling speleothem sections for U-series dating are, in most cases, fairly straightforward due to the presence of visible growth layers. However, not all speleothems possess this property, which depends upon the conditions of calcium carbonate precipitation. In this paper, we describe a method for generating U-series dating samples in which growth layers are resolved from trace-element images produced by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We apply this method to a section of an Italian subaqueous speleothem (CD3) that lacks persistent visible growth layering. The trace-element imaging revealed growth layers that are strongly non-planar in their geometry owing to the speleothem's pronounced euhedral crystal terminations. The most prominent trace-element layers were first digitized as x,y vector contours. We then interpolated these in the growth-axis direction to generate a series of contour lines at 250- m increments. The coordinates of these contours were used to guide the sampling via a computerised micromilling lathe. This produced a total of 22 samples for U-series dating by multi-collector ICP-MS. The dating results returned ages in correct stratigraphic order within error. Close inspection of the U-series data suggests that the main sources of age uncertainty are unrelated to the contour sampling. Comparisons between stable oxygen and carbon isotope profiles derived from aliquots of the dating samples and two other stable isotope profiles from CD3 spanning the same time period compare very favourably. Taken together, this suggests that our trace-element contouring method provides a reliable means for extracting samples for dating (and other geochemical analyses), and can be applied to similar speleothems lacking visible growth layerin

Precise microsampling of poorly laminated speleothems for U-series dating

One of the principal reasons why speleothems are recognised as important palaeoclimate archives is their suitability for accurate and precise uranium-series (U-series) age determination. Sampling speleothem sections for U-series dating is straightforward in most cases because visible growth layers are preserved. However, this is not always the case, and here we describe a sampling strategy whereby growth layers are resolved from trace-element images produced by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We apply this method to a section of an Italian subaqueous speleothem (CD3) that lacks persistent visible growth layering. Trace-element imaging revealed growth layers that are strongly non-planar in their geometry owing to the speleothem's pronounced euhedral crystal terminations. The most prominent trace-element layers were first digitized as x, y vector contours. We then interpolated these in the growth-axis direction to generate a series of contour lines at ∼250-μm increments. The coordinates of these contours were used to guide the sampling via a computerised micromilling lathe. This produced a total of 22 samples for U-series dating by multi-collector ICP-MS. The dating results returned ages in correct stratigraphic order within error. Close inspection of the U-series data and the derived depth-age model suggests that the main source of model-age uncertainty is unrelated to the contour sampling but instead more associated with how closely spaced the model ages are in time, i.e. the model age density. Comparisons between stable oxygen and carbon isotope profiles derived from aliquots of the dating samples and two other stable isotope profiles from CD3 spanning the same time period compare very favourably. Taken together, this suggests that our trace-element contouring method provides a reliable means for extracting samples for dating (and other geochemical analyses), and can be applied to similar speleothems lacking visible growth layerin