4.2 Cloşani Cave
4.2.1 Site description
Cloşani Cave (45.07 °N, 22.80 °E) is located in southwestern Romania at the southern slope of the Carpathians (Figure 4.11). It is developed in massive Upper Jurassic limestone mainly consisting of calcite (93 %) with minor occurrence of dolomite (7 %) (Constantin, 2003).
Drip water and cave air parameters were monitored from 2010 to 2015 (Constantin et al., unpublished data). The deeper sectors of both main passages (Figure 4.12) are characterized by a stable cave climate. Cave temperature shows a mean value of 11.3 °C with inter-annual variations of ± 0.5 °C (Diaconu, 1990). Relative humidity is mostly close to 100 %. The pCO2
of the cave shows a strong annual variability with maximum values of up to 8800 ppm in late
Figure 4.11:Location of Cloşani Cave in Romania (inset) and regional setting of the cave. The cave is located at the southern slope of the Carpathian mountains. Map from Constantin (2003).
Figure 4.12:Internal structure of Cloşani Cave. The cave consists of two main passages, the Crystals Gallery and the Laboratory Gallery. Map from Constantin (2003).
summer and minimum values in winter and spring (1500 ppm). The high pCO2during sum-mer leads to very different conditions for speleothem growth between the seasons, favouring calcite precipitation during winter (Chapter 7).
4.2.2 Climatological framework
The regional climate of the Romanian Carpathians realm is characterized by westerly circu-lation and the influence of the mountain chain topography (Busuioc et al., 2014; Ionita et al., 2014; Micu et al., 2015). By their geographical position within the continent, the Romanian Carpathians lie in an area of interference between five major pressure centers operating over Europe e.g., the Azores Anticyclone, East-European Anticyclone, Mediterranean Cyclones, Icelandic Cyclone and Scandinavian Anticyclone (Micu et al., 2015).
The influence of the Southern Carpathians divides Romania into two major regions of different climatic characteristics by blocking the northerly polar or arctic airflows in winter, as well as of the southerly (tropical) ones. This results in a wetter and colder northern part and drier and warmer conditions in the south. Micu et al. (2015) give a very detailed picture of the air flows and resulting climate patterns in SW Romania.
4.2 Cloşani Cave Annual precipitation in the region of the cave shows two maxima, one from April to June, which is mainly caused by North Atlantic cyclones, and a smaller one between October and December, generated by cyclones originating from the western and central Mediterranean Basin. The winter continental airflows, usually dry and cold, originate in the Arctic Ocean, whereas continental air masses from eastern Eurasia and north Africa or southwest Asia frequently influence the summer precipitation regime with usually very warm and dry spells (Micu et al., 2015). In the region of Cloşani cave, snow fall accounts to about 10 % of total annual precipitation amount, however in the Southern Carpathians frequent snowfalls only occur in areas higher than 1,000 - 1,100 m (Micu et al., 2015).
Large-scale teleconnection patterns
The North Atlantic Oscillation (NAO) - which affects the strength of westerly flow and weather patterns in Europe (e.g. Wanner et al. (2001); Hurrell et al. (2003)) was found to have a strong relationship with winter climate in Romania (Micu et al., 2015). The NAO is the leading mode of climate variability in the North Atlantic region and is characterized by a meridional see-saw in the atmospheric pressure between the Icelandic Low and the Azores High (Wanner et al., 2001). During periods characterized by a positive NAO index westerly flow is enhanced across the North Atlantic and the European region. As a consequence, this phase is associ-ated with stronger-than-average winds across the mid-latitudes of the Atlantic onto Europe and anomalously northerly flows in the Mediterranean region. For example, snow variabil-ity in SW Romania is tightly connected with the NAO (Micu et al., 2015). Moreover, positive thermal anomalies and negative precipitation anomalies in the Romanian area are associated with a high NAO index (Bojariu and Gimeno, 2003; Bojariu and Paliu, 2001).
Another prominent teleconnection pattern over the North Atlantic region is the East At-lantic - West Russia pattern (EAWR, Barnston and Livezey (1987)), which has a strong influ-ence on European climate, especially in the southern and eastern sector. The EAWR was found to have a strong influence over the precipitation in the Mediterranean region (Krichak et al., 2002; Krichak and Alpert, 2005). Extreme wet (dry) winter months are characterized by anomaly patterns which project onto the negative (positive) phase of the EAWR. A strong relationship between the EAWR and the temperature variability inside a cave from the north-western part of Romania has also been found with high (low) temperatures associated with an atmospheric circulation that resembles the center of action of the EAWR pattern (Rimbu et al., 2012). Nissen et al. (2010) found that both NAO as well as EAWR play a significant role on the variability of cyclones and wind activity over the Mediterranean region. The au-thors state, that during the positive phase of the EAWR, the storm numbers over the central Mediterranean region are decreasing and the number of strong wind events over the east-ern Mediterranean region is increasing. Although the EAWR is one of the dominant modes of variability over the North Atlantic-Eurasian region, less attention has been paid to the role of the EAWR pattern on the European climate (Ionita, 2014). The authors investigated the relationship between the EAWR teleconnection pattern and the hydroclimatology of the
European region. The main conclusions of Ionita (2014) are that the EAWR has a strong impact on the coupling between the sub-tropical Atlantic Jet and the African Jet, which in turn affects the climate variability over Europe, and that the strongest impact of the EAWR on precipitation in the central and eastern part of Europe is in mid-winter and early spring.
Additionally, the link between EAWR and European temperature persists from mid-winter to late spring. In general, the link between mid-winter EAWR and European precipitation and temperature was found to be stable in time, however for southwestern Romania, this only applies for winter temperatures and spring precipitation (SPI3, 3-month standardized precipitation index).
Moreover, Comas-Bru and McDermott (2014) concluded, that the relationship between North Atlantic Oscillation (NAO) and winter climate over the North Atlantic - European sec-tor can be linked to the combined effects of the NAO and either the East Atlantic pattern (EA) or the Scandinavian pattern (SCA). In terms of δ18Oof precipitation, the results of Comas-Bru et al. (2016) show, that although the East Atlantic (EA) pattern is generally uncorrelated to δ18O𝑝 during the instrumental period, its polarity affects the δ18O𝑝 - NAO relationship.
In the case of SW Romania, the authors conclude that δ18Omay not be a robust proxy to reconstruct any relationship of the NAO with air temperature or precipitation.
4.2.3 Current state of research
From the southern Carpathian realm, few terrestrial climate records exist despite its impor-tant geographical position as a transitional climatic zone between central Europe and the Mediterranean e.g., Constantin et al. (2007); Feurdean et al. (2008); Onac et al. (2014); Rudzka et al. (2012). Reconstructions based on tree-ring data provide valuable information for the past several hundred years (Levanič et al., 2012; Popa and Kern, 2008), but are limited to the warm season. More recently, guano-derived δ13C-based paleo hydroclimate records made it possible to characterize hydroclimate regimes in Romania during the late Holocene (Onac et al., 2014, 2015; Forray et al., 2015). Low-resolution (i.e. centennial to millennial timescales) stalagmite records, which mainly reflect past variability of winter climate, suggest that the climate in the Carpathian-Balkan region is linked to the North Atlantic region on different time scales (Constantin et al., 2007; Drăguşin et al., 2014; Onac et al., 2002).
Part II
Publications
5 Paper I: Cueva Larga
Millennial scale climate variability on Puerto Rico reconstructed from a multi-proxy speleothem
record spanning 46 - 15 ka BP
Sophie F. Warken1,2∗, Denis Scholz1, Rolf Vieten3, Christoph Spötl4, Klaus P. Jochum5, Thomas E.
Miller6, Amos Winter3,7, Andrea Schröder-Ritzrau2and Augusto Mangini2
1Institute for Geosciences, University of Mainz, Germany
2Institute of Environmental Physics, University of Heidelberg, Germany
3Marine Science, University of Puerto Rico, Mayagüez, Puerto Rico
4Institute of Geology, University of Innsbruck, Austria
5Climate Geochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
6Department of Geology, University of Puerto Rico, Mayagüez, Puerto Rico
7Earth and Environmental Systems Department, Indiana State University, Terre Haute, Indiana, USA
Abstract We present geochemical data from a speleothem from Cueva Larga, Puerto Rico, covering 15 to 46 ka. The chronology is based on precise230Th/U dating. High resolution δ18Ovalues reflect changes in rainfall amount, temperature variability and disequilibrium fractionation effects. The anti-correlation of P/Ca and δ13C values highlights the influence of vegetation and soil productivity on both proxies. In contrast, Mg/Ca, Sr/Ca and Ba/Ca ratios show little variability and are subject to various second order effects. The average δ18Ocomposition suggests that climate on Puerto Rico during the last Glacial was generally cooler by ∼2 - 4 °C and drier than today.
Warm Dansgaard/Oeschger interstadials were characterized by relatively warm and humid conditions. δ18O values during D/O’s 7 and 11 even show comparable conditions to modern climate. In contrast, temperature drops by about 3 - 5 °C and increased aridity are recorded during cold Heinrich stadials, which were partly accompanied by a decline in vegetation and soil productivity. HS 1 was found to be the coolest and driest of all recorded Heinrich stadials. These observations are in agreement with recent research and are ascribed to the southerly movement of the ITCZ and the variable strength of the AMOC.
Our record is the one of the first from this region which clearly shows the impact of both stadials and interstadials on Caribbean climate variability over a relatively long time period. It allows an approximate quantification of last glacial climate conditions.
5.1 Introduction
The reconstruction of tropical hydroclimates has shifted more and more into the focus of paleo climate research (Haug et al., 2001; Cruz et al., 2007; Hodell et al., 2008; Chiang, 2009;
Lachniet et al., 2009). Several studies reveal a strong variability in the tropical hydrologi-cal cycle during the last glacial period closely related to North Atlantic climate variability (Peterson, 2000; Wang et al., 2001; Cruz et al., 2007; Wang et al., 2017).
Ice core and deep sea sediment records of the last 45,000 years show a series of millen-nial scale climate events known as Heinrich stadials (HS) and Dansgaard/Oeschger (D/O) cycles (Dansgaard et al., 1984). Dansgaard/Oeschger events are millennial scale alternations between warm (interstadial) and cold (stadial) periods. The typical cycle has a sawtooth pat-tern, with a very rapid warming event (occurring in a few decades), a slow cooling trend, and then a final fast cooling (Bond, 1997; Wolff et al., 2010).
The so-called Heinrich events are recognized in North Atlantic marine sediments as ice rafted debris (IRD) deposited by icebergs upon melting (Bond, 1997; Heinrich, 1988; Hem-ming, 2004). This freshwater input promoted a slowdown of the Atlantic meridional over-turning circulation (AMOC) and an interhemispheric climate response (McManus et al., 2004;
Wolff et al., 2010; Böhm et al., 2015). Heinrich stadials subsequent to Heinrich events were as-sociated with a reduction in sea surface temperatures (SSTs) in the North Atlantic (Sachs and Lehman, 1999; Hagen and Keigwin, 2002) which was also predicted by model studies (Zhang and Delworth, 2005). This cooling is thought to be caused by reduced northward heat trans-port, driven by the slowdown of the AMOC (McManus et al., 2004; Clement and Peterson, 2008; Böhm et al., 2015), or as suggested by Chiang and Bitz (2005), the increase in sea/land ice. Reduced Northern Hemisphere SSTs led to the southward shift in the Intertropical Con-vergence Zone (ITCZ) and drier conditions in the tropical Northern Hemisphere (Chiang and Bitz, 2005; Zarriess et al., 2011; Stager et al., 2011). Recent studies have also demonstrated the impact of varying height of the Laurentide ice sheet as a climate forcing during Heinrich stadial events (Roberts et al., 2014).
During interstadial periods (i.e. D/O cycles), the inverse occurs with a poleward shift of the Northern Hemisphere summer ITCZ and the jet streams (Asmerom et al., 2010). How-ever, the exact mechanisms driving these events are still not well understood (Clement and Peterson, 2008). The global, abrupt response to Dansgaard/Oeschger interstadials and Hein-rich stadials is recorded in various types of climate archives. For example, the paleo climate records suggest that the global signature of Heinrich stadials includes: a drier Europe (Genty et al., 2003), weaker Asian monsoon (Wang et al., 2001), wetter southwestern North America (Asmerom et al., 2010; McGee et al., 2012), drier northern South America (Peterson, 2000), wetter southern South America (Kanner et al., 2012), an overall drier tropical Asia and Africa (Stager et al., 2011; Deplazes et al., 2014) and a gradually warming Antarctica (Wolff et al., 2010).
While a comprehensive picture of climate responses during North Atlantic Heinrich
sta-5.1 Introduction dials exists from both hemispheres, few studies have been conducted in the (sub-)tropical western Atlantic (Sachs and Lehman, 1999; Grimm et al., 2006; Ziegler et al., 2008; Lachniet et al., 2013; Arienzo et al., 2015; Grauel et al., 2016). Most of these studies either cover only a relatively short time period or show only a selective response to stadials and/or interstadials.
For the northern Caribbean, it is still debated whether the region experienced substantially wetter or drier conditions during Heinrich stadials. Evidence from lacustrine and marine sed-iments from Florida and the Gulf of Mexico suggests, that the northern Caribbean experi-enced warm and wet conditions (Grimm et al., 2006; Donders et al., 2011; Ziegler et al., 2008), in contrast to the southern and southwestern part of the Caribbean basin (Correa-Metrio et al., 2012; Escobar et al., 2012; Lachniet et al., 2013; Deplazes et al., 2013; Grauel et al., 2016).
Moreover, a speleothem record from the Bahamas is interpreted to show cooler temperatures rather than hydrological changes during Heinrich stadials 1, 2 and 3 (Arienzo et al., 2015).
This debate accounts to difficulties in the comparability of the various records, largely be-cause some sites are located in regions which are strongly influenced by multiple effects (e.g., Lachniet et al. (2009)) or because the climate archives reflect different seasonal signals. In addition, uncertainties in chronology and differences in temporal resolution may constitute further complications.
The value of speleothems for paleo climate reconstructions is commonly recognized (e.g., Wang et al. (2001); Fairchild et al. (2006a); Cruz et al. (2007)). Stable isotopes of oxygen and carbon are the most commonly used proxies for speleothem studies (Lachniet, 2009a). In trop-ical locations, the δ18O values of speleothem calcite is typically interpreted to reflect changes in rainfall amount and/or the temperature of the cave (Kanner et al., 2012; van Breukelen et al., 2008).
In Puerto Rico and through-out the Caribbean and south Florida, there is an inverse rela-tionship between the amount of rainfall and the δ18O value of the rainwater (van Breukelen et al., 2008; Scholl et al., 2009) and therefore the amount effect is considered to constitute the main control on the δ18O composition of the rainfall precipitation (Dansgaard, 1964; Lach-niet, 2009a). Also the cave site of this study, monitoring of rain and dripwater over several years confirms that this relationship is also valid for speleothems from this location (Vieten et al., in press).
Consequently, several studies from the Caribbean presented δ18O speleothem records as-sociated with past precipitation variability on various timescales (Lachniet et al., 2009; Win-ter et al., 2011; FensWin-terer et al., 2012, 2013; Lachniet et al., 2013). For instance, WinWin-ter et al.
(2011) related Puerto Rican rainfall with the variability of the Atlantic Multidecadal Oscilla-tion (AMO), as evident from a speleothem δ18O record covering the last 800 a.
Using trace elements as climate proxies is more and more routine in speleothem research.
Elements such as Mg or Sr are well-established as hydrological tracers under certain circum-stances (Cruz et al., 2007; Fairchild and Treble, 2009). Sinclair et al. (2012) developed a model which allows to investigate the most commonly used interpretation of changes in Mg and Sr as being linked to rainfall as a result of calcite-water interactions such as prior calcite
precipitation (PCP).
In this study, geochemical data are presented which are obtained from a speleothem from Cueva Larga, Puerto Rico, spanning the last 46 to 15 ka. Paleo climate reconstruction re-veals substantial changes in the hydrological regime in Puerto Rico associated with Heinrich stadials 1 to 5 and Dansgaard/Oeschger cycles 1 to 12.