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Open Session & Deciphering Clastic Cave Deposits
Surface and underground features in karst regions are rich records of past climatic and environmental changes. This session will showcase the diverse array of studies focused on these records. A partial list of relevant topics includes sea level fluctuations, river incision and base-level oscillations, hydrology and floods, palynology, microbiology, paleontology, petrography and mineralogy, speleogenesis, and the evolution of karst landscapes. There is essentially no restriction on the methods, the archive, or the type of study, as long as it diverges from “classical” paleoclimate time-series based on clastic, speleothem, or cave ice proxies. Multi-disciplinary approaches and novel techniques are strongly welcomed.
Clastic sediments in caves can originate from fluvial, lacustrine, glacial, periglacial, incasion, aeolian, corrosional, and biologic (including anthropogenic) processes. These deposits are more problematic to decipher and date than speleothems. Yet they hold critical information about early phases of speleogenesis, climate, and regional paleohydrology that may be unavailable from other archives. Application of techniques such as paleomagnetism and cosmogenic isotope measurement has allowed determination of conditions going back beyond 6 Ma. The main challenges faced are the discontinuity of many deposits, and the uncertainty of regional fidelity. This session welcomes presentations on all aspects of the deposition, description, and interpretation of clastic deposits in karst settings, with an emphasis on their utility as recorders of environmental conditions. Novel archives and approaches, and especially linkages to the speleothem record, are of special interest.
1:30pm - 1:45pm
Sedimentary records of clastic karst sediments: examples from Slovenia
1Karst research Institute, Research Centre of the Slovenian Academy of Sciences and Arts, Slovenia; 2Institute of Geology of the Czech Academy of Sciences; 3Department of Zoology, Faculty of Sciences, Charles University; 4Institute of Geological Sciences, Polish Academy of Sciences
Cave sediments in karst landscapes are many times the only useful source for studying past environments and processes such as climate changes and tectonic activities. Protected in caves, all cave deposits are well preserved and reveal an exceptionally good, multi-proxy record of surface environmental conditions at the time of their deposition which covers the time span through the last ±20 Ma, thus sometimes their interpretation can be difficult. Cave sedimentation also reflects the evolution of the surrounding landscape; i.e. climatic changes with flood events or changes of the tectonic regimes. Major sources of sediments in caves are allogenic sinking streams, bringing sediments into the underground from eroded noncarbonated rocks. Allogenic cave sediments are particularly important for understanding the environment of their formation before transport and storage in the cave. They are also integrators of extensive river catchments, and have vast chronological potential for understanding past events as well as can give us the insight to the evolution of karst landscapes under influence of allogenic input.
Systematic studies of cave sediments collected from 30 sites in the northwestern Dinarides and the Southern Alps in Slovenia have been ongoing since 1997. Research to date has focused primarily on paleomagnetic studies of allogene sediments in relict and unroofed caves. The results were calibrated, where possible, by U-series, radiocarbon, cosmogenic, paleontological, and geomorphological dating. The calibrated data allowed the development of a robust chronology in the spatially and temporally highly discontinuous preserved sedimentary record. The results indicate that most of the sampled sediments in relict and unroofed caves are up to 5 Ma old, which is contrary to the traditional concept of Pleistocene age for cave sediments in this region. Studies also focused on the importance of sedimentary facies in caves providing indirect evidence of landscape evolution, particularly in catchments - i.e., climatic changes associated with Miocene to Holocene flooding events in tectonically active karst regions. Calibrated data additionally contributed to the interpretation of speleogenesis, deposition in caves, and indirectly to the evolution of karst surfaces and the succession of tectonic regimes.
1:45pm - 2:00pm
Recovering ancient DNA and Neandertal population history from cave sediments
MPI Evolutionary Anthropology, Germany
Over the past few decades, DNA extracted from ancient bones and teeth has played an important role in our understanding of human history. This DNA has revealed interactions between modern humans and Neandertals, large population movements, and the origins of many domestic animals. However, at many archaeological excavations human skeletal remains are absent, preventing a genetic analysis of the individuals and populations that occupied these sites. Even when such remains are recovered, they rarely cover the full time-span of the occupation of a site. Recent advances have demonstrated the recovery of human nuclear and mitochondrial DNA from cave sediments, suggesting that genetic studies of the human past may no longer be tied to the recovery of skeletal elements.
Here, I present an overview of cave sediment DNA, and look to the future of extracting ancient environmental DNA from cave sediments. I will discuss recent work where we recovered ancient human and faunal DNA from cave sediments at sites spanning Eurasia, ranging from 50 to 250 thousand years old. We used this "sediment DNA" to reconstruct human history at these sites, including identifying multiple Neandertal replacement events, which may be tied to contemporaneous changes in climate. We also investigated the use of mammalian sediment DNA as a proxy for taxa abundance, and sought to identify the source of ancient DNA in an analysis of DNA from resin-impregnated micromorphology blocks. However, despite these recent advances, we have much to learn about the interpretation and context of ancient DNA from caves.
2:00pm - 2:15pm
Yucatan aquifer response to climate change (wet-dry) during the Late Holocene; evidence from the Yax Chen and Aktun Ha Cave systems
1Earth, Environment and Society, McMaster University, Canada; 2Applied Mathematics, University of Waterloo, Canada
Our understanding of anchialine hydrology is largely based on short-term instrumental records which are limited spatially and temporally. Short-term interaction between the Meteoric (Me) and Marine (Ma) Water Masses (WM) is fairly well understood, but long-term trends with sea-level and climate change are largely unknown with only limited millennial scale records. Here we use paleolimnological techniques (sediment coring; testate amoebae and foraminifera) and µXRF core scanning (Ti, K, Cl, Si @ 200 µm) of sediment cores in the Yax Chen (Ox Bel Ha; ~ 1-2 km from the coast) and Aktun Ha Cave Systems (~ 8.5 km from the coast), Yucatan, Mexico. These proxies provide high-resolution (< 1 yr) salinity records of the MeWM spanning the past ~ 5 ka showing coincident changes in salinity during wet and dry periods as evidenced in speleothem records from the peninsula. Coincident changes in MeWM salinity show that regional anchialine groundwater systems are broadly sensitive to rainfall. The evidence indicates that during wet periods, rainfall, and flow in the MeWM causes increased input of terrigenous weathering product (Ti, K) and salinity (Cl; microfossils) through entrainment of marine water at the halocline. In contrast, during dry periods, the MeWM is thinner, but there is less flow and entrainment of marine water resulting in lower MeWM salinity. Increased flow during hurricanes and tropical storms (large rainfalls) maybe causing increased salinity of the MeWM during wet periods as evidenced through our instrumental records of Yax Chen. These results have important implications for anchialine aquifers in the Caribbean because small increases in salinity can negatively impact the aquifer and water quality.
2:15pm - 2:30pm
Geochemical, crystal fabric and mineralogic correlations between aragonite and calcite rafts, waters and atmospheric conditions in Cova dets Ases (Mallorca, Western Mediterranean).
1University of the Balearic Islands, Spain; 2University of Zaragoza, Spain
Cova dets Ases is located on the east coast of Mallorca island (Western Mediterranean) and presents a great variety of interesting deposits such as stalactites, stalagmites, POS (Phreatic Overgrowth on Speleothem) or detrital sediments. This work is focused on a specific speleothem deposits called raft. They growth at the air-water interface in brackish pools inside of the cave. The precipitation of these deposits is associated with carbonate supersaturation induced by CO2 outgassing processes at the surface of cave pools. Commonly, they are formed by calcite but, in the cave, they appear as a calcite and aragonite alternation. The principal objective of this work is to establish the relationship among the geochemical, mineralogical and crystal fabric features of the rafts and the environmental conditions (water and atmospheric parameters).
For this propose, water and rafts were sampled monthly during a year at different points in the cavity and atmospheric CO2 concentrations and temperature were also monitored. Raft samples were analyzed isotopically, with X-ray diffraction and observed by SEM. Water samples were analyzed chemically and isotopically.
Information on the interrelationships of precipitated rafts, water and atmospheric conditions will help to understand which are the enabling conditions to raft precipitation, and which are the most suitable conditions for aragonite precipitation. Some of this conclusion can be applied to other speleothem deposits in this and other caves, and will provide new information for paleoenvironmental reconstructions of the western Mediterranean area.
2:30pm - 2:45pm
A quiescent period in Earth’s magnetic field activity recorded in a stalagmite from the mid-to-late Holocene in South America
1Institute for Rock Magnetism, Dept. of Earth & Environmental Sciences, University of Minnesota, USA; 2Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil; 3Department of Geosciences, University of Tübingen, Germany; 4Departamento de Geoquímica, Universidade Federal Fluminense, Brazil; 5Dept. of Earth & Environmental Sciences, University of Minnesota, USA; 6Department of Geology, Trinity College Dublin, Dublin 2, Ireland; 7NASA Goddard Space Flight Center, Greenbelt, USA; 8Instituto de Geociências, Universidade de São Paulo, Brazil; 9Institute of Global Environmental Change, Xi’an Jiaotong University, China; 10Key Laboratory of Karst Dynamics, MLR, Institute of Karst Geology, China
The Earth’s magnetic field is generated at the outer core through convective fluid motions and it is present for most of the Earth’s history. Since 1900, the decay of the dipole intensity has been documented though direct measurements but the past history is only possible through indirect materials. South America has a special feature with the lowest intensity of the geomagnetic field and this is attributed non-dipolar features originated at the core-mantle boundary and it is called South Atlantic Anomaly. A complete understanding of this feature and its past evolution lacks on data throughout the Holocene. One of the materials that have good directional reproducibility and improve data for regions with low spatial coverage are speleothems. These sedimentary cave deposits provide high-resolution records for ages up to 800,000 years by using the U-Th dating method. These records are only possible because magnetic minerals are incorporated into calcite and locked-in during calcite precipitation and generally not subjected to post-depositional processes as generally documented in lakes and marine records. In this study, a stalagmite sample from central Brazil from mid-to-late Holocene, encompassing ~2000 years from 3000 BP to 5300 BP was used to better constrain the geomagnetic field at this location. The high-resolution paleomagnetic results points to a steady variation of the directional and relative paleointensity data, with angular variation < 0.1°/yr., which is not associated with the nondipole terms of the geomagnetic field as observed for the last two millennia. Our record fills an important gap in the reconstruction of the Earth’s magnetic field, depicting a period of a quiescent geomagnetic field during the mid-to-late Holocene in the area that the South Atlantic Anomaly now comprises, and highlights the potential of stalagmites as paleomagnetic recorders.
2:45pm - 3:00pm
Guano corrosion of speleothems
1Jagiellonian University, Poland; 2State Nature Conservancy of the Slovak Republic, Slovak Caves Administration Slovakia; 3Department of Geography, Faculty of Education, Catholic University in Ružomberok, Slovakia
Post-speleogenetic reshaping of cave walls and speleothems can be commonly observed wherever considerable amounts of guano deposits are found. The growth of some speleothems can be punctuated by the episodes of biocorrosion caused by bat guano as well. Considering the high aggressiveness of guano vs calcium carbonate such corrosion appears to be responsible for the loss of a significant portion of paleoenvironmental data. Studies on guano corrosion of speleothems were conducted in caves of the temperate climate zone; Domica and Drienovská Caves in Slovakia, and Nietoperzowa Cave in Poland.
Aggressive nature of guano is a result of the presence of strong and weak acids which are produced during its decomposition. Their production depends upon the astonishingly high content of inorganic and biogenic compounds present in guano. The mixing of guano with cave water showed such a solution has a calcium saturation index between –6.43 and –4.71. The aggressiveness of this solution is indicated by its low pH values which were in a range from 3.37 to 5.85. The measured pH of consolidated guano deposits is usually slightly lower than that of a loose type which commonly occupies external parts of guano heaps.
Limestone and speleothem plates have been installed in the guano heap in Domica and Drienovská Caves to measure the corrosion rate. Their interaction with guano resulted in the development of corrosion pits up to 1 mm in diameter. During almost one year of experiments the mass loss of plates up to 0.916 g was noted, while the maximum corrosion rate was calculated to –26.23 g/cm2/ka. However, the weight of several plates increased; they were covered by pale yellow to dark brown stains and crusts. The observations of speleothems surface affected by guano corrosion showed the presence of analogous coatings. They were composed of ardealite, brushite, and hydroxylapatite.
The interaction with bat guano leads to the formation of remarkably irregular and jagged surfaces which can be observed within speleothems sections. Calcite crystals below these surfaces are highly dissolved and deep pitting corrosion can be observed. In many cases, the corrosion follows the cleavage of crystals. A diagnostic feature of guano corrosion is the precipitation of phosphates and sulphates which usually cover corroded crystals. Moreover, the presence of Mn-oxides has been noted in association with the surfaces affected by guano corrosion.
Geographical ranges of bats’ occurrence indicate that guano corrosion in cave environments can be expected worldwide, especially in subtropical and tropical areas. However, cave inhabitation by bats commonly takes place close to the cave entrance, thus the speleothems in these zones are principally prone to biocorrosion. Regardless of the importance for a paleoenvironmental record of speleothems, the presence of guano-corroded surfaces may serve as an indicator of past bat colonization of the cave.
This research was funded by the Polish National Science Centre (grant no. 2019/35/B/ST10/04397).