Quaternary Science Reviews 20 (2001) 1661}1670 Tyrrhenian Holocene palaeoclimate trends from spelean serpulids Fabrizio Antonioli!*, Sergio Silenzi!", Silvia Frisia# !Environmental Department, ENEA-Casaccia, Via Anguillarese 301, 00060 S.Maria di Galeria Roma, Italy "ICRAM, Central Institute for Marine Research, Via Casalotti, 300, 00163 Roma, Italy #Museo Tridentino di Scienze Naturali, via Calepina 14, 38100 Trento, Italy Abstract In tropical regions, changes in sea level and sea surface temperature (SST) can be obtained from coral reefs. In temperate seas, such as the Mediterranean, where there are neither fossil nor actively growing coral reefs, palaeoclimatic trends and associated sea level rise can be reconstructed through radiocarbon ages and the geochemical properties of serpulid calcite tubes from colonies that dwell in submerged caves. In the present work, we reconstructed Holocene palaeoclimate trends for the Tyrrhenian Sea, through the oxygen isotopic composition of marine Polychaete serpulids that colonised continental speleothems when the sea invaded the caves which open along the Italian coast. The long term !#O trend extracted from multiple serpulid skeletons of &0.7 per mil increase over the last 8 kyr can be interpreted in terms of temperature change, but could also be due to long-term changes in the salinity balance of the Mediterranean. If the !#O trends recorded by serpulid calcite re#ected changes in Mediterranean SST starting at 8200 cal yr BP, it would coincide with the cooling trend recorded in the tropics and in Greenland ice cores. Spelean serpulids radiocarbon ages allowed us to reconstruct a Tyrrhenian sea level curve comparable to that recorded along the French Mediterranean coast. Spelean serpulid calcite, therefore, can be used as a useful tool to reconstruct paleoclimate and sea-level changes in temperate coastal karst regions. ! 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction from carbonate !#O (Keigwin, 1996; Sabin and Pisias, 1996; Bond, 1997), alkenones (Bard et al., 1997), and A knowledge of mid latitude sensitivity to Holocene Sr/Ca ratio in corals (Guilderson et al., 1994; McCulloch global climate changes is essential for modelling how the et al., 1994; Beck et al., 1997) exhibit a marked mid- Earth's climate system responded during the deglaciation Holocene cooling trend interrupted by warm transients. and for simulating future climatic scenarios. Several Ice cores drilled in high mountain glaciers in the high-resolution palaeotemperature records have been Andes (Thompson et al., 1998) show large scale similarit- obtained from high latitude ice cores and tropical coral ies with tropical and polar records, but also regional reefs, foraminifers and glaciers. Climate change recon- di!erences, such as several warm transients in the late struction for the last deglaciation and Holocene, how- Holocene that are not recorded by GISP 2 (Greenland), ever, is still controversial because of di!erent timing and Byrd and Vostock (Antarctica) ice cores. Similarly, the amplitudes of climate variations at high and low latit- record of Holocene climate in the continental Mediterra- udes. For example, the GRIP ice core records are charac- nean region seems to di!er from both the high latitude terised by attenuated !#O trends in the middle and late ice cores and the low latitude marine records (Bar- Holocene that indicate slight cooling for the last 9000 yr Matthews et al., 1997; McDermott et al., 1999). Little is (Dansgard et al., 1993). For the same time slice, the known, as yet, about Holocene sea surface temperature Vostok ice core site in Antarctica records a more accen- (SST) trends in the Mediterranean, as few deposits can tuated cooling interrupted by several warm transients be reliably dated. Marine serpulid overgrowths on sub- (Jouzel et al., 1993). In the tropics, SST changes inferred merged speleothems (CaCO continental cave deposits) which formed in coastal karst regions provide a means of reconstructing the relative timings of sea level rise and paleoclimatic Holocene trends in temperate seas, * Corresponding author. Tel.: #39-6-3048-3955; fax: #39-6-3048- such as the Mediterranean, which are a crucial compon- 4029. ent in understanding the natural variability of climate E-mail address: [email protected] (F. Antonioli). (Bard et al., 1997). 0277-3791/01/$ - see front matter ! 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 1 ) 0 0 0 1 2 - 9 1662 F. Antonioli et al. / Quaternary Science Reviews 20 (2001) 1661}1670 2. Methods tectonically quasi-stable areas of the Italian coast. In fact, the uplift rate following 125 kyr (isotope stage 5e) is In this study we present new C data and oxygen !0.038 mm/yr at Palinuro and !0.014 mm/yr at Ar- isotope time series from Polychaeta serpulid overgrowths gentarola (Antonioli and Oliverio, 1996; Alessio et al., on speleothems sampled in two submerged caves located 1992). Samples were collected by means of scuba diving along the Thyrrhenian coast in Italy at Cape Palinuro to !21 m below sea level in the cave of Argentarola, and (Lat. 40.023N}Long. 15.163E) and at the Island of Argen- down to !48 m in the cave at Palinuro (Fig. 2 ). In the tarola (Lat. 42.263N}Long. 11.073E) (Fig. 1). The two Tyrrhenian Sea, serpulids are typical dwellers of sub- cave sites were chosen on the basis of their location in merged caves, which o!er a dark and sheltered environ- ment, with restricted water #ow. Holocene serpulid colonies grown on submerged speleothems are typically 5}12 cm thick. At the time of sampling, the outermost layer of each colony was alive. Maximum thickness was observed for colonies which grow at the centre of the cave, probably because of the low energy conditions and higher nutrient availability. Optical microscopy observations coupled with scann- ing electron microscopy allowed us to rule out diagenetic alteration of the serpulid tubes and recognise the absence of dissolution voids and abiogenic cements. The growth patterns were also recognized: commonly, new indi- viduals encrust the underlying dead Policheta, thus form- ing a dense colony of intertwined tubes (Fig. 3). Serpulid colonies developed on continental speleothems when the rising sea level reached the speleothem tips. All the colo- nies have been constructed by the gregarious Serpula massiliensis, whose tubes consist of 100% calcite (Milli- man, 1976), as con"rmed by X-ray di!raction. Fossil Fig. 1. The submerged Argentarola cave (Italy) at !18 m. Living serpulids form gregarious colonies on a submerged continental stalag- Fig. 2. Location of the submerged caves along the Thyrrenian coast mite. Maximum thickness of the fossils colonies is 12 cm. (Italy), where speleothems with serpulids overgrowth were sampled. F. Antonioli et al. / Quaternary Science Reviews 20 (2001) 1661}1670 1663 Fig. 4. Main steps of sampling procedure for !#O analysis. Spele- othems with marine overgrowth (A) found in submerged caves were sectioned following the growth lines (B) and analyzed (C). bon dating and one for isotope analysis. A minimum of 6 to a maximum of 21 subsamples (2 mm wide and 2 mm long), were cut from each colony, depending on their growth thickness. Each subsample was powdered (aver- age 4 mg), roasted in vacuum at 3503 for 30 min to pyro- lize organic matter and treated with 100% HPO at 253C for 6 h. The CO released by the reaction and puri"ed by using a liquid nitrogen-ethyl alcohol slash at about !803C, was measured in a Finnigan Delta mass spectrometer. Mean standard deviation of !#O measurements was typically $0.1" (2!). The outermost samples (2 mm), which contained the calcite secreted by living organisms, have a mean !#O value of 2.1$0.2" at both Argentarola and Cape Palinuro. Radiocarbon dating was carried out on 10 colonies, Fig. 3. Close view of a serpulid with inalterated tube (above). Surface 6 at Argentarola cave and 4 at Cape Palinuro in Scaletta layer of a speleothem in the Argentarola cave with serpulids over- cave (Figs. 5 and 6). !#O series were based on 8 colonies, growth (below). while the two colonies coming from Cape Palinuro (!47 and !48 m) were used to build the sea-level curve Serpula massiliensis tubes also consist of 100% calcite, as (Alessio et al., 1996; Antonioli and Oliverio, 1996). Bio- determined by X-ray di!raction. genic samples were not contaminated by speleothem Two-mm wide slabs were cut from 8 serpulid colonies material. The marine and continental deposits are com- perpendicular to the growth axis starting from the living monly separated by a smooth surface and can be easily outer layer down to the crust that "rstly colonised the recognised on the basis of a marked colour di!erence: the continental speleothem by using a diamond drill (Fig. 4). speleothem is translucent and dark brown, whereas the Each slab was then cut into two slices, one for radiocar- marine deposit is opaque and yellowish. 1664 F. Antonioli et al. / Quaternary Science Reviews 20 (2001) 1661}1670 Fig. 5. Section of Argentarola cave showing the location of the sampled speleothems. Fig. 6. Cape Palinuro, section of Scaletta cave with the location of sampled speleothems. The age at which Serpulid colonies commenced grow- bon dating is a slab cut through the whole colony, from ing was calculated by C dating results (Table 1) and by the fossil bottom to the living top. Conventional using a mathematical model that assumes linear growth radiocarbon dating is thus carried out on a sample en- rates. The assumption is also based on the fact that the compassing the whole time span through which the col- colonies do not show any apparent growth hiatus. ony grew. The model allowed us to obtain the age of the The model assumes that growth rates remained con- bottom of the colony.