Available online http://amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 37 (1), 2024, 67-82 ocene Sapropel S1 has been widely identified in the Eastern Mediterranean. Its formation is primarily due to the increased fluvial discharge from the Nile River and the Central Saharan Watershed (Rossignol-Strick et al., 1982; Gallego-Torres et al., 2010; Williams et al. 2015; Blanchet et al., 2021), linked to enhanced monsoonal activity over equatorial Africa, with a partial contribution of increased precipitation in the Mediterranean region including its northern borderlands (Rohling & Hilgen, 1991; Kotthoff et al., 2008; Geraga et al., 2010; Tou- canne et al., 2015; Wu et al., 2017). In the East Mediter- ranean, S1 broadly spans from 10.8 cal ka BP to 5.5 cal ka BP, separated in two distinct phases (S1a and S1b) with a short interruption roughly centered at 8 cal ka BP (Kotthoff et al., 2008; Geraga et al., 2010; Hennekam et al., 2014; Triantaphyllou et al., 2014; Grimm et al., 2015; Filippidi et al., 2016; Wu et al., 2017; Di Donato et al., 2019; Wu et al., 2019). West Mediterranean ORLs could be considered as weakly expressed sapropels (Rogerson et al., 2008), corresponding to anoxic phases, linked to water stagna- tion and enhanced biological production that depletes the available oxygen (Grimm et al., 2015). These anoxic phases may have been originated by hydrological and productivity changes related to the Atlantic inflow from the Gibraltar strait that could have induced the reduction https://doi.org/10.26382/AMQ.2024.04 1. INTRODUCTION The Mediterranean region is particularly influenced by climatic variations due to its semi-enclosed basin, being influenced by the North African subtropical climate and the temperate European westerly atmospheric cir- culation (e.g. Lionello et al., 2006). One of the most important type of paleoclimatic events found in this area is the deposition of reducing layers, mainly identified as Sapropels in the East Mediterranean and as Organic Rich Layers (ORLs) in the West (Rossignol-Strick, 1985; Rohling, 1994; Rogerson et al., 2008; Negri et al., 2012; Fink et al., 2013; Rohling et al., 2015; Incarbona & Sprovieri, 2020; Pérez Asensio et al., 2020). Sapropels are organic-rich layers recovered in marine sediments from many localities throughout the Eastern and central Mediterranean Basin. Kidd et al. (1978) described them as sharply defined, dark colored sedimentary layers with a C org content > 2 wt.% and thickness >1 cm. ORL are defined as dark sediment layers with total organic car- bon (TOC) content up to 1 wt.% (Rogerson et al., 2008; Negri et al., 2012; Fink et al., 2013). Sapropelic layers occur almost periodically in the sediments of the last 13.5 million years in the Mediterra- nean Sea, more often developed in the Eastern than in the Western sub-basins (Rohling et al., 2015). The Hol- EVIDENCE OF ANOXIC LAYERS IN THE CENTRAL TYRRHENIAN SEA BETWEEN 29 AND 4.2 KA Valerio Ruscito 1 , Luisa Conforto 2 , Luigia Manfra 2 , Maria Preite Martinez 2 1 Applied Geophysics Unit GEO-GFI, Geological Survey of Italy, ISPRA, Rome, Italy. 2 Department of Earth Sciences, Sapienza University of Rome, Rome, Italy. Corresponding author: Valerio Ruscito ABSTRACT: Heavy metals contents are analyzed in a continuous sediment record spanning from 29 to 4.2 ka BP, sampled on the upper continental slope facing the Ombrone River mouth (Tyrrhenian Sea) and investigated in previous works by isotopic and micropaleontological studies supported by radiocarbon dates. The aim of this work is to use changes in some heavy metals con- tents, which were shown to reflect change in the pH and Eh on the sea floor environment, to investigate the mechanism involved in the expression of bottom anoxia between 29 and 4.2 in the Central Tyrrhenian Sea. Fe, Mn, and Pb content shows significant correlations with paleoenvironmental changes. Pb elemental concentration proved to be the most powerful tool to reconstruct envi- ronmental redox conditions, being greatly enhanced during strong anoxia phases and showing conservative behavior not influ- enced by post depositional diagenesis. Geochemical evidence from Ombrone submerged delta sediment suggest the occurrence of two reducing phases. During the Holocene, data highlight the presence of a reducing layer in the West Mediterranean between 7.4 and 5.7 ka BP, with a climax between 6.8 and 5.7 ka BP, in agreement with previous evidence. In this period, probably, en- hanced river runoff increased the flux of continental organic matter into the basin and reduced sea surface salinity, leading to the formation of an anoxic environment. The presence of this reducing layer agrees with evidence of a “pluvial” period recognized by other authors in the West-Central Mediterranean, partially synchronous with the Sapropel S1 formation in the East, suggesting its regional character and its extension to the whole Mediterranean Basin. Additionally, trace metal data suggest the occurrence of an additional anoxic phase, not previously recognized, and occurring during the Last Glacial Period, at about 27.2 ka BP. Keywords: Reducing Layers; heavy metals; West Mediterranean Sea; Last Glacial-Middle Holocene period; paleoclimatology; inorganic geochemistry. mailto:valerio.ruscito@isprambiente.it of the deep circulation (Rohling et al., 2015). They also could derive from an enhanced river runoff, following the deglaciation and the alpine ice melting, or from an in- creasing precipitation regime that caused a decrease in surface salinity (Rogerson et al., 2008; Rohling et al., 2015; Grant et al., 2016; Bakrač et al., 2018; Bazzica- lupo et al., 2018; Pasquier et al., 2019). The great influ- ence of the North Atlantic climate on western Mediterra- nean hydrological activity is demonstrated by many other authors (Bard et al., 2002; Zanchetta et al., 2007; Toucanne et al., 2015; Pasquier et al., 2019; Wagner et al, 2019). Late Glacial-Holocene anoxia periods, leading to ORL deposition, are found in the Alboran Sea, spanning from 15 to 8.5 ka BP (Rogerson et al., 2008; Fink et al., 2013; Jimenez-Espejo et al., 2015; Dubois-Dauphin et al., 2017; Bazzicalupo et al., 2018), in the Gulf of Lion (Pasquier et al., 2019) and in the Ligurian Sea spanning from 15 to 11 ka BP and from 11 to 6 ka BP (Le Houedec et al., 2021). All these studies prove an im- portant enhance of freshwater runoff from northern bor- derland in the west Mediterranean Basin. At present the runoff in this region mainly depends on the North Atlan- tic atmospheric circulation and is affected by local orog- raphy of the coasts (Trigo et al., 2002; Kandiano et al., 2014). The present work focuses on climatic changes recorded in several cores collected in the Arcipelago Toscano Basin, in the Central Tyrrhenian Sea, near the Ombrone River mouth (Fig. 1). One of the core was previously investigated by oxygen isotope analyses on foraminifera and micropaleonotological analyses, to- gether with radiocarbon age determinations (Belluomini et al., 2002; Carboni et al., 2005). The aim of this work is to use changes in some heavy metal contentsome heavy metalselago Toscano b, which were shown to reflect change in the pH and Eh on the sea floor environment, to investigate the mecha- nism involved in the expression of bottom anoxia be- tween 29 and 4.2 in the Central Tyrrhenian Sea (Fig. 1). (Thomson et al., 1995; Warning & Brumsack, 2000; Naimo et al., 2005; Reitz et al., 2006; Tribovillard et al., 2006; Angelidis et al., 2011; Heimbürger et al., 2012; Oliveri et al., 2013; Jiménez-Espejo et al., 2015; Mar- tinez-Ruiz et al., 2015). This approach is suggested by previous works (Conforto & Manfra, 2004; Conforto & Manfra, 2007; Conforto et al., 2008), that evidenced significant correlations between heavy metals behaviors and paleoclimatic changes. New geochemical analyses, reported in this work, together with a review of previous geochronological, isotopic, sedimentologic and micro- paleontological results, allowed a more detailed pale- oenvironmental reconstruction of this area. 2. STUDY AREA AND MATERIAL The studied area (Fig. 1) is comprised between the 200 m isobath and the Tuscany coast, in the Arcipelago Toscano National Park area. It is limited on the south by the Argentario Mount and Giglio Island, extending north- ward to Punta Ala and Elba Island in the Tuscan mining district (Elba Island and Metalliferous Hills on the main- land). The continental shelf receives terrestrial sediment supply mainly discharged from the Ombrone River (monthly min/max mean discharge 4/50 m3/s) (Bellotti et al., 2004). The role of the minor rivers (Bruna and Alben- ga) is subordinate. The Ombrone River is 161 km long and drains a basin of 3496 km2 characterized by phyl- lites, quartzites, arenites, siliceous and carbonate se- quence, Plio-Pleistocene clayey-sandy sediments, ophi- olitic sequences and trachyandesite volcanics (Bellotti et al., 2004). At present, the Ombrone mouth is a wave- dominated delta consisting of fine sand to clay sedi- ments (Tortora et al., 1999; Bergamin et al. 2001). Coarser fluvial sediments are distributed mainly north- ward by longshore currents, while finer ones drape the shelf (Aiello et al., 1975; Tortora, 1999). This area was shown to be ideal for Quaternary paleoclimatology studies based on geological and geo- chemical analyses (Belluomini et al., 2002; Carboni et al., 2005), due to its geomorphological condition of a semi-enclosed basin. A submerged structural high, called Elba Ridge, delimits the shelf to the western side and has a great impact on the morphologic and sedi- mentary setting of the area, both presently and during the late Quaternary sea-level lowstand (Roveri & Cor- reggiari, 2004). Geochemical analyses were performed on the Z145 core, located on the upper continental slope at about 22 km SW off the Ombrone River mouth, at a depth of 150.8 m (Fig. 1). During the Last Glacial Maxi- mum (LGM), the continental shelf was largely exposed, and the distance between the Z145 sampling site and the Ombrone mouth was only about 7 km (Alessio et al., 1994). During the last deglaciation, the sea level rise and the following stabilization (3.5-3.0 ka BP) caused the eastward migration of the Ombrone river mouth, and a lagoonal delta system developed (Carboni et al., 2005). Z145 core is about 4 m long and spans continuous- 68 Ruscito V. et al Fig. 1 - Map of the study area and samples locations. Black dot indicates the core Z145 (150 m depth, Lat. 42°32’15” N, Long. 10°47’47” E), sampled on the upper continental slope about 22 km SW off the present-day Ombrone River mouth. Open trian- gles show the eight box-cores sampled on recent sediments of the continental shelf. 69 Evidence of anoxic layers in the Central Tyrrhenian Sea between 29 and 4.2 ka. Fig. 2 - a) Lithological log of core Z145 and samples analyzed in this study (● ). b) Sedimentation Rate (SR) is calculated using measured radiocarbon dates. Modified from Carboni et al. (2005). ly from about 29 to 4.2 cal ka BP. To know the present heavy metals background in the area, analyses were also carried on recent sediments sampled in 8 box- cores (A4, A5, A7, B60, B6m, F12, I12 and A15f), col- lected on the continental shelf (Fig. 1). Core B60 is the site nearest to Z145 and represents a good candidate to study the recent sediments at the top of the Z145, that were lost during sampling. The Z145 sediment consists mainly of grey mud in the upper part of the core, and of an alternation of mud- dy and sandy mud levels below 180 cm. The detailed log and sedimentological features of Z145 core are re- ported in Carboni et al. (2005), together with isotopic (δ18O on foraminifera) and micropaleontological results. The record is anchored to an age model supported by seven radiocarbon dates (Carboni et al., 2005). The sedimentation rate (SR) in Z145 is very low during the Glacial period, sharply increases in the Younger Dryas and reaches the highest value at about 7.5 cal ka BP, when anoxia conditions established (Fig. 2). The eight small cores of recent sediments are about 15 cm long (A4, A5, A7, B60, B6m, F12, I12, A15f) and are mainly composed of silty clay. They are dated be- tween 1950 and 1992 by 137Cs and 210Pb methods (Belluomini et al., 2002). 3. PREVIOUS PALEOCLIMATIC EVIDENCES The previous analyses carried on Z145 core allow the paleoclimatic reconstruction for this area. Basing on micropaleontological and isotopic evidence, Carboni et al. (2005) identify the Glacial period in the bottom lay- ers, and the LGM at about 26 cal ka BP (Tab. 1 and Fig. 3, dates recalibrated in this work), according to Ivy-Ochs et al. (2008), McCullock et al. (2010), Shakun et al. (2010), Adamson et al. (2013), Domnguez-Villar et al. (2013), Sarikaya et al. (2015), Oliva et al. (2018) and Seguinot et al. (2018). Z145 proxies identify also the Younger Dryas cold event, spanning from about 12.9 cal ka BP to 11.7 cal ka BP, according to several authors (Ivy-Ochs et al., 2008; McCullock et al., 2010; Fiedel, 2011; Carlson, 2013; Fink et al., 2013; Kennet et al., 2015; Beyin et al., 2017; Bazzicalupo et al., 2018; Gromig et al., 2018; Keigwin et al., 2018; Oliva et al., 2018; Pauly et al., 2018; Ribolini e al., 2018). A cooler period is highlighted between 8.8 and 7.4 cal ka BP, synchronous with the cooling event recorded at 8.2 cal ka BP, according to several authors (Rohling & Pälike, 2005; Spötl et al., 2010; Magny et al., 2011; Rodrigo-Gàmiz et al., 2011; Lirer et al., 2013; Schem- mel et al., 2016). A warm and wet period with stratified oligotrophic waters occurred from 7.4 cal ka BP until about 5.2 cal ka BP. Micropaleontological data suggest anoxic condi- tion, likely related to a hard rainy phase, spanning about 1000 years, established on the borderland of western Mediterranean basin, as recognized in Croatia (Bakrač et al., 2018) and in SW Spain (Schröder et al., 2018). After the anoxic event, bottom water oxygenation re- established, and climatic conditions changed in a drier and cooler period corresponding to the top of the core and placed at about 4.2 cal ka BP, in agreement with 70 Tab. 1 - Z145 radiocarbon dates (from Carboni et al., 2005, modi- fied using Reimer et al., 2020 calibration curves). a = AMS dates; b = LSC dates; M.P. = Median Probability. Dated control points are shown in bold and dates in italics are extrapolated and approximat- ed to 10 years. In the first column, asterisks (*) indicate the chemi- cally analyzed levels. Geochemical data: δ18O of G. bulloides (from Carboni et al., 2005). Ruscito V. et al 5. RESULTS AND DATA INTERPRETATION Table 1 report δ18O data and radiocarbon ages determination for core Z145, presented in Carboni et al. (2005). Tables 2-6 show concentrations values of Fe, Mn and Pb of the analyzed samples from Z145 core, used for the paleoclimatic reconstruction, and from the 8 box-cores (Fig. 1), which provide information about the recent background of these elements in the area. Great significance has the box-core B60 because it is the site nearest to Z145 and represents a good candidate to study the top sediments lost during Z145 sampling. Our data allow a detailed study of geochemical behavior of Pb, Mn and Fe in order to obtain a better reconstruction of paleoenvironmental conditions in the Ombrone delta submerged area. Concerning the gen- eral behavior of elements in various pH and Eh condi- tions see Glasby (2006), Mihaljevic (1999), Williamson (1999). Overall, variations in Pb, Mn and Fe content stud- ied in the Z145 record are in good agreement with the previously identified climatic phases (see Sec.3) from the last Glacial Period until 4.2 cal ka BP, allowing often a better delimitation of their boundaries (Fig. 3). 5.1. Pb In core Z145, Pb shows highly enhanced values both in the Glacial and the Holocene periods. During the Holocene, the distribution of Pb (Tab. 2a and Fig. 3a) shows a concentration increase between 6.8 and 5.7 cal ka BP, with a peak zone mean of 246 ppm, reaching its maximum at 6.2 cal ka BP (332 ppm). These values are clearly anomalous: the peak value is about 13 times higher than the mean (26 ppm) of the other values dur- ing the Holocene. Moreover, these values are about six times the B60 mean (56 ppm, Tab. 3), and about nine times the present background in the area (35 ppm, Tab. 4). In these levels, the sedimentation rate (SR) (Fig. 2) increases to 44 cm/ka, while at the beginning of the Holocene it was about 22 cm/ka (see Fig. 3a). Overall, there is not a strict correlation between Pb variation and several authors (Melki et al., 2009; Colonese et al., 2013; Lirer et al., 2013; Di Rita et al., 2018; Schröder et al., 2018; Bini et al., 2019; Isola et al., 2019). During the mid-Holocene period, Magny et al. (2011), Colonese et al. (2013) and Di Rita et al. (2018) identify a paleoclimatic limit, between the SE/NW Medi- terranean regions, placed at 40-42° N, characterized by wetter winters in the northern areas and humid summers in the southern. As the Z145 core is situated at 42° 35’N, the study area is likely mainly representative of the cli- matic situation of the Northern Mediterranean region, perhaps with some influences from the Southern. 4. MATERIALS AND METHODS Analyses of Fe, Mn and Pb concentrations were performed on 17 sediment samples from core Z145 and on 8 samples from the box-cores, using an ICP-AES housed in the Earth Science Department of the Sapien- za University (Rome). Instrumental errors mainly range between 1-15%. Total concentrations were determined following a strong hot acidic attack using mixtures of perchloric and hydrofluoric acids. A sequential extraction to measure metals partitioning into the main fractions was performed following the method proposed by Sahu- quillo et al. (1999). In order to improve the knowledge of the elements behaviors in different environmental condi- tions, we directly measured concentrations in the car- bonate, oxides and organic partial fractions, while the residual content has been determined as difference. Not all oxides, mostly the Fe oxides, could be dissolved in this sequential analysis (Fletcher, 1981), the metals easily removable (i.e. adsorbed on the clay minerals) are comprised in the carbonate fraction. Radiocarbon dates of core Z145 (from Carboni et al., 2005) are recalibrated using the marine calibration curve IntCal20 (Reimer et al., 2020) and expressed as calendar marine ages with a reservoir age correction of 400 yrs. Isotopic values are reported following new ages calibration. (Tab. 1). For the micropaleontologic data see Carboni (2005). 71 Fig. 3 - Heavy metals concentration in core Z145. a) Pb concentration curve. b) Fe/100 and Mn concentration curves, their data are report- ed using different scales. c) G. Bulloides δ18O values from Carboni et al (2005). Points mark the analyzed samples (see Tab. 1). The grey areas show the Younger Dryas and reducing climax periods, the grey line indicates the Last Glacial Maximum (LGM). Pb and Fe recent background are reported. Mn mean value is calculated from Glacial and Holocene Z145 means. Evidence of anoxic layers in the Central Tyrrhenian Sea between 29 and 4.2 ka. SR trend throughout the entire record, indicating that probably Pb anomalies are not primarily driven by in- creases in inland sedimentary contribution. The high Pb values probably are due to environmental conditions with low redox potential (Eh), where the bacterial activity reduces the sulfates to sulfides, leading to the precipita- tion of Pb as PbS (Zhang et al., 2014; Turner et al., 1986). PbS is not influenced by post depositional reoxi- 72 dation, as its solubility product is the lowest, after HgS, among metallic elements sulfides. The reducing condi- tions are probably due to an enhanced freshwater input that, together with following reduced sea surface salinity highlighted by the isotope data, hampered the bottom water levels oxygenation. This is in good agreement with Drab (2015), who found heavy metals sulfides increase in two sapropels in the Marmara Sea. Tab. 2 - Heavy metals concentrations in core Z145. a) Holocene samples. Top record does not coincide with the seafloor as the top sedi- ments were lost during sampling. b) Glacial samples. Ages are calculated from control points (Tab. 1). Tab. 3 - Pb, Fe/100, Mn concentrations in B60 box-core. Tab. 4 - Pb, Fe, Mn mean values in recent samples (box-cores). The last column shows the present background of the area, calculated from all of the sites means. Tab. 5 - Pb, Mn, Fe percentage distributions in the partial fractions in core Z145 and in box-core B60. In Z154, level at 108 cm corresponds to the reducing layer, level at 215 cm corresponds to the beginning of the Holocene. Ruscito V. et al carbonate. At the -108 cm level its values reach about 90% in the residual phase, being only 10% in the organ- ic one, and close to zero in the two other phases. These values support the notion of an anoxic environment, causing a remarkable Pb precipitation as PbS. During chemical sequential extraction, PbS is dissolved after the oxidizing step, with following precipitation as PbSO4 enriched in the residual phase (Cappuyns et al., 2007). This evidence confirms the attribution of the level at - 108 cm to an anoxic period, when reducing conditions were strong enough to hamper the presence of Pb in the oxide and carbonate phases. In the older level at - 215 cm, Pb shows higher values in the organic and oxide phases, suggesting a presence of a more oxidized environment. In B60 (Tab. 5, Fig. 4c), Pb concentrates mainly in the organic and residual fractions and this distribution differs from the Z145 samples. This is probably due to the diagenesis of the organic matter in the older Z145 sediments. Pb partition in the oxide and carbonate frac- tions shows not very significant differences between the two cores, considering that absolute values are about only a few ppm. All data suggest that Pb is a very sensitive proxy to Another high Pb concentration value is also meas- ured during the Late Glacial, at about 27.2 cal ka BP (Tab. 2b and Fig. 3a). Here, Pb reaches 309 ppm, a value close to higher Holocene contents and about 13 times higher than the mean of other Glacial values (24 ppm). Environmental conditions in the study area were very different between the two periods. During the Gla- cial, the Ombrone River flowed into the Tyrrhenian Sea several kilometers closer to the Z145 site, increasing the supply from the coastal environment, although the hy- drological cycle and the river runoff could have been weaker than during Holocene times. Although only one sample is here available, because of the low sedimenta- tion rate, as Pb value is very similar to the Holocene ones, we can hypothesize also in this glacial interval the presence of a reducing layer. The Pb behavior in the partial fractions from cores Z145 and B60 is also investigated (Tab. 5). In Z145, values are measured at-108 cm, corresponding to the reducing layer, and at -215 cm, corresponding to the beginning of the Holocene (Fig. 4d). Pb mainly concen- trates in the residual (88% at - 108 cm and 57% at -215 cm) and organic (10 at - 108 cm and 34% at -215 cm) fractions and to a lesser extent in the oxides and in the 73 Fig. 4 - Pb and Mn percentage distribution in partial fractions: a) Mn in B60; b) Mn in Z145; c) Pb in B60; d) Pb in Z145. Evidence of anoxic layers in the Central Tyrrhenian Sea between 29 and 4.2 ka. identify environmental sapropel-type conditions, high- lighted by Pb concentrations reaching high anomalous values. It must be emphasized that Pb content in these layers is about 9 times the present background, even though this area in the last century has been affected by anthropic pollution, mainly from mining activities and lead-added fuel. Evidence confirms that the anomalous concentrations in Z145 were exceptionally high com- pared to present background values, suggesting that Holocene, and probably Glacial, reducing layers condi- tions were strongly different than the present. 5.2. Mn During the Holocene, in the Z145 core Mn concen- tration has a mean value of 852 ppm and increases toward the top of the core (Tab. 2a, Fig. 3b). This fea- ture is due to a remobilization of Mn in the deeper, more reducing, levels and its upward diffusion and re- oxidation (Petrie, 1999). During sedimentation indeed Mn is stripped from the sediments and moves to the surficial layers (Callender, 2003; Glasby, 2006). In re- ducing conditions, Mn oxyhydroxide precipitation is in- hibited (the lowest Mn value, during this phase is meas- ured at 6.6 cal ka BP) enhancing the concentration of Mn+2 dissolved in pore water. Above reducing levels, higher Mn precipitation occurs during more oxidizing environmental conditions. The subsequent bioturbation of the superficial sediments brought downwards O2 rich waters that oxidized dissolved Mn, causing to the for- mation of a secondary Mn peak at 6.2 cal ka BP, still within the anoxic layer previously identified (Thomson, 1995; Reitz et al., 2006; Rey et al., 2008; Angelidis et al., 2011; Martinez-Ruiz et al., 2015). During the Glacial period (Tab. 2b, Fig. 3b), de- spite the lower sedimentation rate which causes a scarcer resolution in the sedimentary record, there is evidence of some Mn variations mostly due to glacial climatic fluctuations (Svensson et al., 2008; Guilizzoni et al., 2014; Oliva et al., 2018) that influenced the environ- mental and geochemical parameters (depth, distance from coast, temperature, organic substance, river sup- plies, pH, Eh, etc.). Tab. 2b and Fig. 3b show two Gla- cial Mn maximum values (28.2; 22.4 cal ka BP), sepa- rated by a minimum at 26 cal ka BP, synchronous with the LGM. Mn behaves similarly during the Holocene reducing phase, but its re-oxidation begins slightly after the end of the anoxic period identified by high Pb val- ues. This delay is probably due to the location of the site nearer to the coast during the Glacial period, where the lower pH values hampered the Mn precipitation until the environment became oxidizing enough (Glasby, 2006). Moreover, as “true” marine geochemical conditions (Eh, pH, etc.) were not fully attained throughout the Glacial period, there is not an increasing trend upwards, as showed by the Glacial Mn mean which is only 558 ppm, much lower than in the Holocene (852 ppm). In recent sediments of core B60 (Tab. 3), Mn reaches very high values in the top levels due to its remobilization from deeper levels and its upward diffu- sion and re-oxidation. During the Holocene, Mn behav- iors in B60 and in Z145 are similar, but in B60 the oxida- tion process is stronger because the sampled levels are found in the more oxygenated surficial layers at the water-sediment interface, whereas the top levels of core Z145 were lost during sampling. Mn geochemical behavior is also evidenced by comparing its mean concentration in B60 to the Mn means at other sites of recent deposition (see Tab. 4). Mn means can be roughly grouped into three different areas (Fig. 5). In the first area (B60 and I12) Mn reaches its highest values. In the inner area (A15f, A7 and F12) Mn has the lowest concentrations of the whole basin. In the central area (A4, A5, and B6m), Mn has intermediate concentrations. Mn distribution therefore depends on the distance to the coast. A15f, A7 and F12 are more influ- enced by coastal environment and the river inputs (pH, Eh, wave energy, sedimentation, etc.) and have low Mn precipitation. B60 and I12, conversely, are in more ma- rine environment where Mn oxide precipitation is fa- vored. A4, A5, and B6m in the intermediate environment show intermediate concentration values. This pattern is consistent with the two different Mn features in Z145 during Glacial and Holocene times, when the distance to coast varied (Glasby, 2006). In level -108 cm (corresponding to the Holocene reducing layer) and in level -215 cm (corresponding to the beginning of the Holocene), Mn reaches the lowest concentrations (6-9%) in the organic fraction and it shows similar values (24-31%) in the residual fraction (Tab. 5, Fig. 4b). The sum of percentage values of the oxidized and carbonate fractions is nearly constant (63- 67%), the Mn distribution in each fraction being influ- enced by its oxidation state (Mn +2 in carbonate phase and Mn +4 in oxi-hydroxides minerals). At -108 cm (6.7 cal ka BP), Mn is mainly concentrated in the carbonate phase, in agreement with the recognized environmental anoxic conditions that hampered the formation of Mn oxides (Petrie, 1999; Callender, 2003; Glasby, 2006). It must be emphasized that during the anoxic period the environmental reducing conditions were strong so that the Mn value in the oxide phase is below the detection limit. Conversely, at -215 cm (about 10.9 cal ka BP) the 74 Fig. 5 - Map of Mn mean concentration areas in recent sedi- ments of the study area. Blue area = Mn concentration between 2500 and 1500 ppm. Green area = Mn concentration between 1500 and 800 ppm. Yellow area = Mn concentration between 800 and 500 ppm. Ruscito V. et al marine conditions are attained, partially hampering the precipitation of the Mn. The mean Fe concentration in B60 (Tab. 3) is about 12% less than the Holocene mean in Z145 (Tab. 2a). A similar decrease (about 15%) is also recogniza- ble in all other recent sediments (Tab. 4), probably due to the presence of continuous iron mining activity in the Tuscan mining district since the Etruscan age. Mineral exploitation reached the maximum during the Industrial Age and had ended by the mid-twentieth century. Min- ing probably caused significant decrease in the total average Fe content in the environment. In both Z145 and B60 (Tab. 5), Fe is mainly con- centrated in the residual fraction (95-99%), and it is only represented in small percentages in the organic and oxide portions. The low presence of the oxide fraction is probably due to the chemical-reducing attack that is not sufficiently strong to dissolve the iron oxides. All the Fe, then, is nearly attributed to the residual phase. 6. DISCUSSION Chemical analyses performed in this study, togeth- er with previous isotopic and micropaleontological stud- ies, allow the identification of two reducing layers depos- ited in the Central Tyrrhenian Sea. Anoxic conditions occur thanks to the favorable position of the sampling site within the continental platform, at about 150 m depth during high-stand periods, facing three river mouths, in a basin where water exchanges are weak- percentage of Mn+4 in the oxide fraction is higher than the percentage of the Mn+2 in the carbonate phase. This feature corresponds to “normal” oxidized sedimentation in a not fully marine environment, as the sea level was yet lower than the present and the sampling site was nearer to the coast. In B60, as in Z145, most of Mn concentrates in carbonate and oxide fractions with nearly constant val- ues of about 86-90%. In the superficial level, the Mn value in the oxide fraction reaches 41%, sharply de- creasing in the sub superficial levels. Below the level - 1.5 cm, Mn mainly is in the reduced form (Mn+2), strongly enriched in the carbonate fraction, in agreement with fully marine condition (Glasby, 2006). 5.3. Fe In core Z145, the Fe and Mn trends (Fig. 3b) are similar, due to the redox conditions found in this deposi- tional environment (Williamson, 1999; Callender, 2003; Haese, 2006; Martinez-Ruiz et al., 2015). Before the Glacial-Holocene transition their values strictly correlate (r=0.9) and are still in good agreement during the first phase of the Holocene (r=0.8) (see Fig. 6 a, b). In Gla- cial and Holocene anoxic periods, data show very low Eh values inducing the reduction of Fe. Mn and Fe dis- solve and follow the same trend. Similar correlations are found also by Olivieri et al. (2013) in anoxic sediments. After the end of the Holocene reducing layer, the onset of oxidizing conditions caused a progressive difference between the Fe and Mn trends (Fig. 3b): the Mn concen- trations increase toward the top of the core, while Fe does not follow this trend. These trends also occur in recent sediments, such as in B60 and F12. In core B60, there is no correlation between Mn and Fe (Tab. 3 and Fig. 6c), because the elements have different behaviors in oxidizing conditions. Mn-Fe, instead, show a weak correlation (r=0.6) in F12 (Fig. 5d and Tab.6), due to the localization of this site near to the coast where not fully 75 Tab. 6 - Mn and Fe/100 concentrations in box-core F12. Fig. 6 - Fe-Mn correlations in Ombrone cores. a) Fe-Mn correlation in Z145 during the Glacial period. b) Fe-Mn correlation in Z145 during the Holocene period. c) Fe-Mn correlation in B60. d) Fe-Mn correlation in F12. Evidence of anoxic layers in the Central Tyrrhenian Sea between 29 and 4.2 ka. ened by few submerged highs such as the Elba ridge. In this situation, the influence of the enhanced continental runoff, mostly of the Ombrone River supply, was strong enough to create reducing environments, leading to some sapropel-type levels. 6.1. Holocene Reducing Layer Former micropaleontological and isotopic analyses (Carboni et al., 2005) and new trace element data agree in highlighting the presence of a reducing layer in the studied area during the Holocene. This anoxia period is characterized by phase of enhanced productivity (Carboni et al., 2005), with an increase of SR (Fig. 2b) from 17 to 44 cm/ka. The TOC content rises from about 0.5-0.6% up to about 1% (Ruscito personal communica- tion), a typical value of West Mediterranean ORLs (Rogerson et al., 2008; Fink et al., 2013). Faunal assemblages highlight a change in the marine circulation regime from well-mixed eutrophic to stratified oligotrophic waters in a temporal range span- ning from about 7.4 to 5.2 cal ka BP (Carboni et al., 2005). The lighter δ18O values (Tab. 1, Fig. 3) mark the increased freshwater input from 7.1 cal ka BP to slighlty after 5.7 cal ka BP. At about 6.8 cal ka BP, the reducing conditions became strong enough to produce a remark- able Pb precipitation and its concentration shows a tem- poral range of enrichment comprised between 6.8 and 5.7 cal ka BP, peaking at 6.2 cal ka BP (Tab. 2a, Fig. 3a). Low Mn values (Fig. 3b, Tab. 2a) suggest the pres- ence of reducing conditions at ~6.7 cal ka BP, while the concentration peak at 6.2 cal ka BP probably does not coincide with the end of the anoxia period, but rather represents a post depositional re-oxidation of the sedi- ment by bioturbation (Rey et al., 2008), as high Pb con- centration shows that reducing conditions were present at least until 5.7 cal ka BP. This difference of about 500 years corresponds to ~ 20 cm of reworked sediment, in agreement with Reitz et al., (2006). The ranges of all proxies mostly overlap, defining the chronological boundaries of the ORL. Pb shows the narrower time span, highlighting the climax of the period between 6.8 and 5.7 cal ka BP. Among our investigated parameters, the Pb concentration, not influenced by post- depositional re-oxidation, is acknowledged to be the best marker to detect the climax of the anoxic period. This period is almost synchronous with the Sapro- pel layer S1b (7.4-6.4 cal ka BP) found by Bakrač et al. (2018) in Lake Vrana (Croatia) corresponding to an increased freshwater discharge, and also with the maxi- mum level at Lake Medina (6.2-5.8 cal ka BP) found in the SW Iberian Peninsula by Schröder et al. (2018). It also partially overlaps with a main increase of Alpine floods found by Sabatier et al. (2017), thus highlighting its regional significance over the Mediterranean Basin. Our data agree with the enhanced rainy period recognized by many authors in the West Central Medi- terranean area (Zanchetta et al., 2007; Tinner et al., 2009; Spötl et al., 2010; Zhornyak et al., 2011; Fletcher et al., 2013; Toucanne et al., 2015; Bakrac et al., 2018), taking into account some delay between the increase of precipitations on the continental sites and the develop- ment of oligotrophic and reducing conditions on the sea floor (Di Donato et al., 2019). Holocene anoxia period found in the Ombrone submerged delta also partially overlaps with the last phases of Sapropel S1 found in the East Mediterranean (Grimm et al., 2015; Grant et al., 2016; Tesi et al., 2017; Swingedown et al., 2019; Wu et al., 2019), showing that enhanced freshwater runoff during sapropel formation was not restricted to the Eastern Mediterranean but was rather widespread over the entire Mediterranean region, including its northern borderlands. Moreover, it suggests that in the West Mediterranean region this “pluvial” peri- od reached its maximum during the last phases of sap- ropel S1b formation in the East. In the Ligurian basin (North Tyrrhenian Sea) in the 11-6 ka period, enhanced rivers activity and low ventilat- ed bottom sea waters occurred. Moreover, during the last 6 ka, a windy cooler climate improved the bottom water reoxygenation (Le Houedec et al., 2021), in good agreement with our paleoclimatic reconstruction from the Ombrone submerged delta. 6.2. Glacial Reducing Layer Despite the scarcer resolution due to the low sedi- mentation rate (Fig. 2b), at about 27.2 cal ka BP another anoxic period, of similar intensity of the Holocene one, was hypothesized basing on trace element data. Here high Pb concentrations are found, and Pb reaches 309 ppm (Tab. 2b and Fig. 3a), a value close to that observed in the Holocene anoxic layer, about 13 times higher than the mean of other Glacial values (24 ppm). This suggests that chemical and physical condi- tions were similar. Based on an estimated sea level of about 120-130 m below the current one (Alessio et al., 1994; Capozzi et a., 2009; Clark et al., 2009), during the Glacial period the distance between the Ombrone River mouth and the sampling site was about 7 km and the water depth about 20-30 m. This setting emphasized the coastal supplies, although the hydrological cycle and the river runoff were probably weaker than during the Holo- cene. The enhanced Pb concentration at 27.2 cal ka BP is synchronous with a phase marked by the increase of the warm-water species G. bulloides (Carboni et al., 2005) and by decreasing of δ18O ratios (Tab. 1, Fig. 3). These data highlight a warmer and wetter fluctuation during the Late Glacial (Svensson et al., 2008; Guiliz- zoni et al., 2014; Oliva et al., 2018). This climatic amelio- ration probably induced an increased freshwater supply that produced in our area a stratified column with reduc- ing conditions, involving Pb precipitation. 8. CONCLUSIONS New trace element data performed on marine sedi- ments collected on the Ombrone submerged delta (Central Tyrrhenian Sea), complement the previously published paleoenvironmental reconstruction in the area between 29 and 4.2 cal ka BP and allows the recon- struction of changing oxidation condition at the sea bot- tom. Fe, Mn and Pb contents proved to be powerful tools for recognizing periods of anoxia in the basin, be- ing particularly sensitive to environmental variations of pH and Eh conditions. We found that Pb was the most 76 Ruscito V. et al tin, 62, 1041-1052. Doi: 10.1016/j.marpolbul.2011.02.030 Bakrač K., Ilijanić N., Miko S., Hasan O. (2018) - Evi- dence of sapropel S1 formation from Holocene lacustrine sequences in Lake Vrana in Dalmatia (Croatia). Quaternary International, 494, 5-18. Doi: 10.1016/j.quaint.2018.06.010 Bard E., Delaygue G., Rostek F., Antonioli F., Silenzi S., Schrag D.P. (2002) - Hydrological conditions over the western Mediterranean basin during the depo- sition of the cold Sapropel 6 (ca. 175 kyr BP). Earth and Planetary Science Letters, 202, 481- 494. Doi: 10.1016/S0012-821X(02)00788-4 Bazzicalupo P., Maiorano P., Girone A., Marino M., Combourieu-Nebout N., Incarbona A. (2018) - High-frequency climate fluctuations over the last deglaciation in the Alboran Sea, Western Mediter- ranean: Evidence from calcareous plankton as- semblages. Palaeogeography, Palaeoclimatology, Palaeoecology, 506, 226-241. Doi: 10.1016/j.palaeo.2018.06.042 Bellotti P., Caputo C., Davoli L., Evangelista S., Garzanti E., Pugliese F., Valeri P. (2004) - Morpho -sedimentary characteristics and Holocene evolu- tion of the emergent part of the Ombrone River delta (southern Tuscany). Geomorphology, 61 (1- 2), 71-90. Doi: 10.1016/j.geomorph.2003.11.007 Belluomini G., Branca M., Improta S., Manfra L., Ruscito V., Vesica P., Voltaggio M. (2002) - Il sistema deltizio del Fiume Ombrone: determinazione delle velocità di sedimentazione mediante metodi radio- metrici. Studi Costieri, 5, 35-45. Bergamin L., Celia Magno M., Chiocci F.L., Di Bella L., La Monica G.B., Landini B. (2001) - Stratigrafia dell’immediato sottofondo marino antistante il fiu- me Ombrone. Conference Bilancio sedimentario dei sistemi costieri italiani. Processi naturali ed influenze antropiche, Porto d’Ischia, May, 15-17, 2001. Abstract Book. Beyin A., Prendergast M.E., Grillo K.M., Wang H. (2017) - New radiocarbon dates for terminal Pleistocene and early Holocene settlements in West Turkana, northern Kenya. Quaternary Science Reviews, 30, 1-18. Doi: 10.1016/j.quascirev.2017.04.012 Bini M., Zanchetta G., Persolu A., Cartier R., Català A., Cacho I., Dean J.R., Di Rita F., Drysdale R.N., Finnè M., Isola I., Jalali B., Lirer F., Magri D., Masi A., Marks L., Mercuri A.M., Peyron O., Sadori L., Sicre M-A., Welc F., Zielhofer C., Brisset E. (2019) - The 4.2 ka BP Event in the Mediterranean Re- gion: an overview. Climate of the Past, 15, 555- 577. Doi: 10.5194/cp-15-555-2019 Blanchet C.L., Osborne A.H., Tjallingii R. Ehrmann W., Friedrich T., Timmermann A., Brückmann W., Frank M. (2021) - Drivers of river reactivation in North Africa during the last glacial cycle. Nature Geoscience, 14, 97-103. Doi: 10.1038/s41561-020-00671-3 Callender E. (2003) - Heavy metals in the environment: sensitive proxy to identify environmental sapropel-type (i.e. anoxic) conditions, with total concentrations reach- ing high anomalous values (about 9 times the present background) during specific past periods, even though this area in the last century has been affected by an- thropic pollution, mainly from mining activities and lead- added fuel that increased the natural Pb background. In agreement with previous data, two reducing periods have been recognized. The first occurs during the Glacial at about 27,2 cal ka BP. The second occur during the Holocene between 7.4 and 5.2 cal ka BP, with a climax between 6.8 and 5.7 cal ka BP, identifying a sapropel-type sedimentation phase. The formation of the anoxic layer in the Ombrone submerged delta during the glacial period was favored by the geomorphological con- ditions of a semi-enclosed basin, where water exchang- es with the open sea were weakened. Formation of this layer is also related to the increased vicinity of the site to the river mouth during the marine low-stand, that al- lowed increased freshwater supply. The Holocene reducing layer was probably a con- sequence of an intense rainfall period in the West- Cen- tral Mediterranean area, representative of the climatic situation of the Northern Mediterranean region, perhaps with some influences from the Southern part of the re- gion. The Holocene anoxic period partially overlaps with the last phases of Sapropel S1b found in the East Medi- terranean, suggesting that the humid climate during sapropel formation was not restricted to the Eastern Mediterranean, but was rather widespread over the en- tire Mediterranean region, although the highest rainfall phases were not strictly synchronous in the two basins. The comparison with this paleoenvironmental re- construction in a period not affected by human activity could help to evaluate anthropic influences in current environmental conditions and to make predictions of future developments. ACKNOWLEDGEMENTS We would like to thank Prof. F.L. Chiocci, who pro- vided us with samples collected as part of the CNR ma- rine campaigns. We also thank Prof. B. Landini and Dr C. Tarragoni for their aid in the sample classification. Dr. Voltaggio and another anonymous reviewer are acknowledged for their positive comments to the first versione of the ms. 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