Stratigraphic evidence for a “pluvial phase” between ca 8200–7100 ka from Renella cave (Central Italy)

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Abstract

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 Mediterranean.

Introduction

Many palaeoclimate records from the central and eastern Mediterranean region suggest a wetter climate during the Early to Middle Holocene compared with the second half of the Holocene. These include stable isotope studies of lacustrine carbonates, marine cores and speleothems (Kallel et al., 1997, Bar-Matthews et al., 2000, Zanchetta et al., 2007a, Zanchetta et al., 2007b, Roberts et al., 2008), and pollen-based studies from lakes (e.g. Jahns and van den Boogard, 1998, Rossignol-Strick, 1999, Sadori and Narcisi, 2001, Kotthoff et al., 2008). Further south in the Sahara region, which is currently the world’s largest warm desert, the wettest conditions for the Holocene (the so-called “African Humid Period”: Gasse et al., 1990, Gasse, 2000, deMenocall et al., 2000, Kuper and Kröpelin, 2006) were also experienced during the first half of this epoch. At this time, monsoon circulation was enhanced, and led to a substantial increase in Nile River discharge. This increase in discharge is considered to be one of the main causes of stagnation in the eastern Mediterranean Sea, leading to the formation of the organic-rich sapropel layer, S1 (e.g. Rossignol-Strick et al., 1982, Rohling, 1994, Krom et al., 2002, Scrivner et al., 2004). Stagnation of the eastern Mediterranean may also have been enhanced by increased river discharge derived from local rainfall (Kallel et al., 1997, Bar-Matthews et al., 2000, Kotthoff et al., 2008).

The degree of synchronicity between the interval of enhanced precipitation throughout the eastern and central Mediterranean on the one hand, and sapropel formation (i.e. increased monsoon activity at lower latitudes over Africa, Rohling et al., 2002) on the other hand, has yet to be determined. This is principally because of the difficulties of synchronizing terrestrial and marine records. Speleothem records from Soreq Cave (Israel) (Bar-Matthews et al., 2000) and Corchia Cave (central Italy) (Zanchetta et al., 2007b) both show a period of enhanced precipitation between ca 9 and 7 ka, which was possibly related to an increase in the contribution of vapour sourced from the North Atlantic (as opposed to the Mediterranean) due to increased westerly circulation (Kallel et al., 1997, Bar-Matthews et al., 2000). This is broadly consistent with the timing of the incursion of Mediterranean-sourced rainfall into the Red Sea between ca 9.5 to 7.3 ka cal BP (Arz et al., 2003), which further suggest increased westerly circulation. However, De Lange et al. (2008) recently showed that the eastern Mediterranean basin was predominantly oxygen-free below ca 1800 m water depth for a ∼4700-year period of synchronous, basin-wide sapropel S1 formation between ca 10.8–6.1 ka cal BP. If these chronologies are correct, this implies that rainfall sourced from the Atlantic reached its maximum between ca 9 and 7 ka BP, whereas enhanced summer monsoon precipitation over the Nile catchment, responsible for much of the increased freshwater discharge into the Eastern Mediterranean Sea and formation of Sapropel S1, may have occurred for a longer period.

Despite general agreement for of a wetter period during the Early Holocene across the Mediterranean region, some discrepancies prevail between archives. Roberts et al. (2008), in reviewing stable oxygen isotope records from lakes in the Mediterranean basin, suggested that there is sufficient evidence for an Early Holocene humid period in the eastern to central Mediterranean sector but not in the western sector. In addition, some lake-level records in the central to eastern Mediterranean suggest low-stand conditions for a significant part of the Early-Middle Holocene (Giraudi, 1998, Migowski et al., 2006, Magny et al., 2007), in apparent conflict with the concept of a humid Early Holocene. An interesting case is central Italy, where an interval of significantly depleted 18O values between ca 8.9 and 7.3 ka from stalagmite CC26 (Corchia Cave) has been interpreted as a phase of enhanced rainfall over the Apuan Alps (Fig. 1) (Zanchetta et al., 2007b). This 18O-depleted interval started slowly, with the lowest δ18O values reached at ca 8 ka, then ended abruptly at ca 7.3 ka. However, during the first part of this period, the nearby lakes of Accesa and Fucino (e.g. Giraudi, 1998, Magny et al., 2007, Fig. 1) experienced low-stand phases. Although this discrepancy could be reconciled considering the different residence times of water in the different hydrological systems, and the relative sensitivity of each site to changes in the seasonality of precipitation, as recently suggested Giraudi et al. (2010), further evidence is needed to improve the general framework of climate evolution at this time, both at local and regional (Mediterranean-basin) scales, and the relationship with events responsible for S1 formation.

Recently, Spötl et al. (2010) noted that the wettest phase identified at Corchia (i.e. ca 8 to 7.3 ka) was synchronous with a period of enriched δ18O in southern alpine and eastern alpine speleothems, interpreted as an increase in moisture sourced from the Mediterranean relative to the otherwise more predominant northwesterly air masses that affect the Alps.

In this paper, we report stratigraphic and chronological evidence from Buca della Renella (Renella Cave) which shows evidence for increased clastic sediment input (i.e. fluvial activity) in the cave at the time of the wetter conditions inferred from the Corchia speleothem δ18O record. Although the cave experiences seasonal fluvial invasion, the higher passage upon which this study is focused only receives flows during very large storms. Thus, periods when speleothem growth is interrupted within this upper chamber provide insights into past intervals of ‘regular’ flood activity in the area and thus increased regional rainfall.

Section snippets

Site description

Renella Cave (44°05′42″ N, 10°11′01″ E) is a small, shallow cave located at the confluence of Canale Regolo and the Frigido River (Alpi Apuane karst) near the city of Massa in central Italy (Fig. 1). The region experiences a Mediterranean climate, with a predominantly North Atlantic influence, and a mean annual precipitation of ∼2000 mm. The mean annual temperature is ca 13 °C while mean monthly temperatures range from 23.1 °C in July to 7.4 °C in January. The surface above the cave has a

Materials and methods

Flowstone samples from different stratigraphic levels were collected for radiometric dating to constrain the timing of phases of high-flood activity. A particular focus was the succession of two phases of speleothem formation observed in the upper (now inactive) chamber (Fig. 2). These flowstones are separated by alluvial sands (Fig. 3, Fig. 4). Using a micro air-drill, a total of 17 samples weighing 20–30 mg were extracted from segments of two flowstones (RL18, RL13, Fig. 4) for U–Th dating.

Results and discussion

Reconstructing cave sediment stratigraphy is complex (e.g. Fig. 7.2 in Fairchild et al., 2007), especially without the aid of robust chronological constraints. Although a U/Th chronology can be applied successfully on relatively pure calcite precipitates (e.g. Hellstrom, 2006), direct dating of calcites associated with phases of clastic sediment deposition can be problematic (Stock et al., 2005, William and White, 2007). Consequently, constraining the age of alluvial phases, which likely

Conclusion

A stratigraphic and chronological study of the infilling of the upper level of Renella Cave reveals a period of increased frequency of flooding in the upper, now inactive, passage, which was particularly severe between ca 8.2 and 7.1 ka, and which left thick layers of sand. This period of enhanced flood activity was substantially in phase with the interval of lowest δ18O values recorded in a stalagmite from nearby Corchia Cave, inferred to have resulted from an increase in local precipitation.

Acknowledgments

We thank the Federazione Speleologica Toscana and Parco delle Apuane for their continuous logistical support in studying Apuan Alps caves. This work was financially supported by University of Pisa and the Australian Research Council (DP0773700). We thank the editor, M. Bar-Matthews, and an anonymous reviewer for their pertinent comments which allowed us to improve the quality of the manuscript.

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