Miocene biochronology and paleoceanography of the North Pacific

Biostratigraphic correlation based on microfossil datum levels, directly or indirectly tied to the paleomagnetic time scale, provides a high resolution time control for the Miocene in the equatorial and middle latitude North Pacific. Faunal changes and abundance fluctuations of planktic foraminiferal species combined with the oxygen Pacific. Faunal changes and abundance fluctuations of planktic foraminiferal species combined with the oxygen isotope record of foraminifers, reveal the paleoclimatic and paleoceanographic history. The planktic foraminiferal assemblage change in the early Miocene, extinction of Oligocene fauna and rise of a highly diverse Neogene fauna, appears to be related to increased water mass stratification in the world oceans presumably resulting from the establishment of circum-Antarctic circulation. An increase in the siliceous productivity in the eastern equatorial Pacific region between 20 and 18 Ma suggests that the vertical and horizontal circulation was intensified at that time. Climates cooled rapidly during the middle Miocene between 14 and 13 Ma suggesting the growth of a major east Antarctic ice sheet. Paleoclimatic conditions remained generally cool, although oscillating, during the late Miocene. In the late early to middle Miocene faunal provincialism developed between low and middle latitudes, and by late Miocene time a distinct provincialism similar to the present was established.     

Middle Miocene paleoceanography of the western equatorial Pacific (DSDP site 289) and the evolution ofGloborotalia (Fohsella)

Evolution of the planktic foraminiferal lineageGloborotalia (Fohsella) occurred during the Miocene between 23.7 and 11.8 Ma and forms the basis for stratigraphic subdivision of the early middle Miocene (Zones N10 through N12). Important morphologic changes within theG. (Fohsella) lineage included a marked increase in test size, a transition from a rounded to an acute periphery, and the development of a keel in later forms. We found that the most rapid changes in morphology ofG. (Fohsella) occurred between 13 and 12.7 Ma and coincided with an abrupt increase in the δ¹⁸O ratios of shell calcite. Comparison of isotopic results ofG. (Fohsella) with other planktic foraminifers indicate that δ¹⁸O values of the lineage diverge from surface-dwelling species and approach deep-dwelling species after 13.0 Ma, indicating a change in depth habitat from the surface mixed layer to intermediate depth near the thermocline. Isotopic and faunal evidence suggests that this change in depth stratification was associated with an expansion of the thermocline in the western equatorial Pacific. After adapting to a deeper water habitat at 13.0 Ma, theG. (Fohsella) lineage became extinct abruptly at 11.8 Ma during a period when isotopic and faunal evidence suggest a shoaling of the thermocline. Following the extinction ofG. (Fohsella), the ecologic niche of the lineage was filled by theGloborotalia (Menardella) group, which began as a deep-water form and later evolved to an intermediate-water habitat. We suggest that the evolution ofG. (Fohsella) andG. (Menardella) were tightly linked to changes in the structure of the thermocline in the western equatorial Pacific.     

Neogene Pacific microfossils and paleoceanography

Data on temporal and spacial distribution of all species of foraminifera, calcareous nannofossils, radiolaria and diatoms have been compiled from the Initial Reports of the Deep Sea Drilling Project, for the Neogene Pacific Ocean. Problematic species and sites were removed by a series of procedures. Age-depth curves for each site, based on all available biostratigraphic information, were used to choose narrow time-slices for analysis. Factor analysis was applied to the resulting data body to derive paleo-oceanographic and -ecologic information. For this data set, radiolaria were found to be the single most useful group, and calcareous nannofossils the least. Use of all groups combined delineates major provinces better than the use of any one group alone.From the Early Miocene to the present the number of plankton provinces has increased while the geographic extent of each has decreased, coincident with steepening of surface isotherms. The Tropical Province shows relatively little geographic change, but high diversification of microfossil groups. The Transitional Provinces have been remarkably stable both geographically and biologically since their appearance in the Middle Miocene, reflecting the highly adaptive nature of the component species. Subarctic and Subarctic Provinces have existed throughout the Neogene, representing the positions of the Subarctic Front and the Antarctic Convergence. The former has been generally of low diversity throughout time, while the latter has increased in diversity of siliceous organisms, and become progressively more restricted. An Antarctic Province appears in the Middle Miocene, and also shows change in species composition and geographic restriction through time. A Bering Province has appeared since the Miocene, probably in response to refrigeration of the Arctic in the Pliocene.     

Radiolarian faunal turnover across the Oligocene/Miocene boundary in the equatorial Pacific Ocean

The global warming trend of the latest Oligocene was interrupted by several cooling events associated with Antarctic glaciations. These cooling events affected surface water productivity and plankton assemblages. Well-preserved radiolarians were obtained from upper Oligocene to lower Miocene sediments at Ocean Drilling Program (ODP) Leg 199 Sites 1218 and 1219 in the equatorial Pacific, and 110 radiolarian species were identified.Four episodes of significant radiolarian faunal changes were identified: middle late Oligocene (27.5 to 27.3 Ma), latest Oligocene (24.4 Ma), earliest Miocene (23.3 Ma), and middle early Miocene (21.6 Ma). These four episodes approximately coincide with increases and decreases of biogenic silica accumulation rates and increases in δ¹⁸O values coded as “Oi” and “Mi” events. These data indicate that Antarctic glaciations were associated with change of siliceous sedimentation patterns and faunal changes in the equatorial Pacific.Radiolarian fauna was divided into three assemblages based on variations in radiolarian productivity, species richness and the composition of dominant species: a late Oligocene assemblage (27.6 to 24.4 Ma), a transitional assemblage (24.4 to 23.3 Ma) and an early Miocene assemblage (23.3 to 21.2 Ma). The late Oligocene assemblage is characterized by relatively high productivity, low species richness and four dominant species of Tholospyris anthophora, Stichocorys subligata, Lophocyrtis nomas and Lithelius spp. The transitional assemblage represents relatively low values of productivity and species richness, and consists of three dominant species of T. anthophora, S. subligata and L. nomas. The characteristics of the early Miocene assemblage are relatively low productivity, but high species richness. The two dominant species present in this assemblage are T. anthophora and Cyrtocapsella tetrapera. The most significant faunal turnover of radiolarians is marked at the boundary between the transitional/early Miocene assemblages.We also reviewed changes in other microfossil assemblages in the low latitudes during the late Oligocene through early Miocene. The microfossil assemblages of major groups show sequential changes near the Oligocene/Miocene (O/M) boundary (23.8 Ma). Many extinction events and some first occurrences of calcareous nannofossils and many occurrences of radiolarians are found from about 24.8 to 23.3 Ma, and first occurrences of planktic foraminifers and diatoms followed from 23.2 through 22 Ma. Hence, the O/M boundary is identified as a significant level for microfossil evolutions.     

Annual and seasonal changes in diets of martens: evidence from stable isotope analysis

Theory predicts that generalist predators will switch to alternative prey when preferred foods are not readily available. Studies on the feeding ecology of the American marten (Martes americana) throughout North America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp.). We investigated seasonal and annual changes in diets of martens in response to the changing abundance of small rodents (Peromyscus keeni, and Microtus longicaudus) on Chichagof Island, Southeast Alaska, using stable isotope analysis. We hypothesized that martens would feed primarily on small rodents during years with high abundance of these prey species, whereas during years of low abundance of prey, martens would switch to feed primarily on the seasonally available carcasses of salmon. We also hypothesized that home-range location on the landscape (i.e., access to salmon streams) would determine the type of food consumed by martens, and martens feeding on preferred prey would exhibit better body condition than those feeding on other foods. We live-captured 75 martens repeatedly, from mid-February to mid-December 1992–1994. We also obtained marten carcasses from trappers during late autumn 1991 and 1992, from which we randomly sub-sampled 165 individuals. Using stable isotope ratios and a multiple-source mixing model, we inferred that salmon carcasses composed a large portion of the diet of martens in autumn during years of low abundance of rodents (1991 and 1992). When small rodents were available in high numbers (1993 and 1994), they composed the bulk of the diet of martens in autumn, despite salmon carcasses being equally available in all years. Selection for small rodents occurred only in seasons in which abundance of small rodents was low. Logistic regression revealed that individuals with access to salmon streams were more likely to incorporate salmon carcasses in their diet during years of low abundance of small rodents. Using stable isotope analysis on repeated samples from the same individuals, we explored some of the factors underlying feeding habits of individuals under variable ecological conditions. We were unable to demonstrate that body weights of live-captured male and female martens differed significantly between individuals feeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist predators, switched to alternative prey when their principal food was not readily available on a seasonal or annual basis. Although salmon carcasses were not a preferred food for martens, they provided a suitable alternative to maintain body condition during years when small rodents were not readily available. Received: 1 May 1996 / Accepted: 24 February 1997     

Specific variability in Fe-enriched cultures from the equatorial Pacific

Water collected on the spring equatorial Pacific Joint GlobalOcean Flux Study (JGOFS) was placed in ultra-clean bottles inan incubator on deck under the direction of the team of J.H.Martin.Half of the bottles were enriched with 2.53 mM iron; the otherhalf served as controls. Cell counts increased, while the numberof species was reduced, in both the controls and the treatedsamples over the period of the experiment. Diatoms dominated,and after an early growth spurt in the Fe-enriched bottles,most diatom species showed greater growth in the controls atthe end of the 6 day experiment. However, as the experimentwas terminated, more cells overall were noted in the Fe-enrichedsamples, with the most abundant diatom, Cylindrotheca closterium.principally responsible for the difference. Growth rates werehigh in both the controls and the treated samples, with overallrates of 1.3–3.0 divisions day^–1 in the Fe-treatedsamples and 1.9–3.4 divisions day^–1 in the controls.The group of smaller pennate diatoms averaged 2.9 divisionsday^–1 in the Fe-treated samples and 3.3 divisions day^–1in the controls over the entire experiment.     

Late Miocene—early Pliocene planktonic foraminiferal biostratigraphy and paleoceanography of low-latitude marine sequences

Plate reorganization and development of polar glaciation are closely associated with the changing climatic conditions of the Cenozoic. The planktonic foraminiferal fauna in three low-latitude DSDP sites (224, Ninety East Ridge; 317B, Manihiki Plateau; 366A, Sierra Leone Rise) has been examined to determine how these regions responded to the late Miocene climatic cooling that has been previously observed in high-latitude regions. It has been proposed that an expansion of Antarctic glaciation and a resultant eustatic regression are associated with this cooling, with the latter being at least partially responsible for the Messinian “salinity crisis” in the Mediterranean.Only one of the sites, 366A, shows any significant faunal change during the late Miocene and early Pliocene (approximately 8-3 Ma). During the late Miocene, there is a decrease in the abundance of species considered to be tropical-subtropical, suggesting the incursion of a cool water mass into the Sierra Leone Rise region during this time. The lack of any major faunal changes at Sites 214 and 317B indicates that the water masses of the low-latitude regions of the Indo-Pacific were relatively unaffected by this cooling in the late Miocene.The three sites also show no evidence for a change in the level of the CCD between the late Miocene and early Pliocene; however, this is most likely due to their equatorial position and shallow water depth. Higher-latitude sites from both the Atlantic and the Pacific reveal a definite shoaling of the CCD during the late Miocene.     

东太平洋中部中上层鲨鱼群落营养生态位分化

鲨鱼在大洋生态系统中占据着重要的生态地位,其作为顶级捕食者,通过下行效应直接影响生态系统的稳定.稳定同位素技术是目前研究摄食生态学强有力的手段之一,可利用碳氮稳定同位素在食物网中的特性分别指示鲨鱼的食物来源和营养级.本研究选取8种130尾采集自东太平洋中部的中上层鲨鱼,应用稳定同位素绘制其种群生态位图谱,比较不同种群间的生态地位及资源分配方式上的差异.结果表明:不同鲨鱼种群碳、氮稳定同位素比值存在显著差异;8种鲨鱼在东太平洋生态系统中的营养级为4.3~5.4,大青鲨、尖吻鲭鲨与其他6种鲨鱼存在摄食隔离,表现出独特的营养生态地位.这些结果充分证明大洋性中上层鲨鱼并非生态系统的冗余种,其营养生态位的独特性不会被其他捕食者简单地替代和弥补.     

Diet differences among age classes of Arctic seals: evidence from stable isotope and mercury biomarkers

A basic understanding of current food web dynamics and baseline data from which to measure future change is necessary to understand species re-distribution and altered competition for food with climate change. We use mercury (Hg) and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios as biomarkers to understand species diet differences and age class differences among ringed (Phoca hispida), bearded (Erignathus barbatus), and harbour (P. vitulina) seals in a subarctic marine ecosystem. Adult bearded seals had significantly lower δ¹⁵N and muscle Hg than bearded seal pups, whereas the opposite was observed in ringed seals where pups had lower δ¹⁵N than adults, suggesting age specific foraging differences in trophic food level for both species. For harbour seals, δ¹⁵N did not differ significantly among ages while Hg and δ¹³C did. The δ¹³C in muscle supports that bearded seals in this study are benthic feeders and are part of a separate food web from ringed seals and harbour seals. Harbour seals had the highest levels of mercury and δ¹⁵N, indicating they feed at a higher trophic level relative to the other two seal species. Carbon and nitrogen isotopic ratios and Hg levels illustrate how resources are partitioned among three seal species and offer evidence for separation based on life stages within species.     

Early Pleistocene glacial-interglacial radiolarian assemblages from the eastern equatorial Pacific

Polycystine radiolaria from ODP Hole 677A in the eastern equatorialPacific were examined at isotopically identified Early Pleistoceneglacial maxima and minima. Two distinct radiolarian assemblagesare recognized, characterizing glacial and interglacial optima.The Glacial Assemblage is characterized by high abundances ofTheocalyptra davisiana, Botryostrobus auritus, Anthocyrtidiumzanguebaricuim andHexacontium enthacanthum. The InterglacialAssemblage is characterized by Tetrapyle octacantha, Octapylestenozoa and Theocorythium vetulum. A comparison of these fossilassemblages with modern radiolarian distribution suggests thatthe Glacial Assemblage represents intensified upwelling of coldadvected water via the Eastern Pacific Boundary Current, whilstthe Interglacial Assemblage indicates climatic ameliorationin the eastern equatorial Pacific, with the prevalence of warm(>21C) tropical/subtropical surface waters. The recognitionof these radiolarian assemblages could be successfully appliedto studies of adjacent east Pacific areas where other palaeoecologicalindicators are lacking.     

Dietary and environmental reconstruction with stable isotope analyses of herbivore tooth enamel from the Miocene locality of Fort Ternan, Kenya

Tooth enamel of nine Middle Miocene mammalian herbivores from Fort Ternan, Kenya, was analyzed for δ¹³C and δ¹⁸O. The δ¹⁸O values of the tooth enamel compared with pedogenic and diagenetic carbonate confirm the use of stable isotope analysis of fossil tooth enamel as a paleoenvironmental indicator. Furthermore, the δ¹⁸O of tooth enamel indicates differences in water sources between some of the mammals. The δ¹³C values of tooth enamel ranged from −8·6–−13·0‰ which is compatible with a pure C₃diet, though the possibility of a small C₄fraction in the diet of a few of the specimens sampled is not precluded. The carbon isotopic data do not support environmental reconstructions of a Serengeti-typed wooded grassland with a significant proportion of C₄grasses. This study does not preclude the presence of C₃grasses at Fort Ternan; it is possible that C₃grasses could have had a wider geographic range if atmospheric CO₂levels were higher than the present values.     

Integrated stratigraphy and paleoceanography of the Messinian (latest Miocene) across the North Atlantic Ocean

The Messinian was a time of major climatic and paleoceanographic change during the late Cenozoic. It is well known around the Mediterranean region because of the giant anhydrite/gypsum sequence and the suggested desiccation of the Mediterranean Sea. However, this interval is less constrained outside the Mediterranean region, where several paleoceanographic changes could have taken place because of the desiccation. Hence, we present an integrated stratigraphic framework for future Messinian paleoceanographic studies, determination of the effect of the Mediterranean desiccation on deep-water paleoceanography, and comparison of intra-Mediterranean paleoceanographic changes with those in the open oceans during the Messinian Stage.Four DSDP/ODP Holes (552A, 646B, 608, and 547A) from the North Atlantic Ocean and one land borehole from Morocco have been studied to integrate bio-, magneto-, and stable isotope Messinian stratigraphy with high resolution sampling. Our results produce the best assessment of the Tortonian/Messinian boundaries in all holes because they do not rely on any one signal.In paleomagnetic Subchronozone CSAn.lr in the Salé borehole and DSDP Site 609, a S/D coiling direction change of Neogloboquadrina pachyderma/acostaensis appears to indicate PMOW entering the northeastern Atlantic Ocean, at least reaching 50°N. Diachrony and synchrony of some important Messinian planktic foraminifera from these Atlantic DSDP/ODP holes and the Salé borehole, such as the LO of Gq. dehiscens, the LO of Gt. lenguaensis, the FO and LO of Gt. conomiozea, the FO of Gt. margaritae s.s., the FO of Gt. puncticulata, and the FO of Gt. crassaformis are discussed for understanding some of the paleoceanographic changes. This integrated stratigraphie framework presented here allows much better North Atlantic correlations at this critical point in Messinian geologic history.     

Fish reduce habitat coupling by a waterbird: evidence from combined stable isotope and conventional dietary approaches

Aquatic consumers can function as habitat couplers by using allochthonous subsidies of prey that migrate across ecosystem boundaries. We examined the relative use of allochthonous (invertebrates—terrestrial or living on littoral vegetation; immigrating amphibians) versus autochthonous (aquatic invertebrates, fish) resources by the red-necked grebe Podiceps grisegena, a generalist predator, on fishless ponds versus ponds stocked with common carp Cyprinus carpio. We combined conventional methods of diet estimation with stable carbon and nitrogen analyses of egg yolks and putative prey of grebes. Prelaying grebes were observed to take mainly adult amphibians on fishless ponds and fish on stocked ponds. Alimentary tract analyses gave more weight to invertebrate prey, especially leaf beetles Donaciinae, apparently picked off water or emergent plants. Bayesian isotopic mixing models did not reveal predominance of a single food source but indicated that in the presence of fish grebes received relatively less energy for egg formation from amphibians and leaf beetles. Overall, our results show that grebes relied more on allochthonous resources (range of means 50–97 % of the biomass contribution estimated by different assessment methods) in the absence than in the presence of fish (8–23 %). We suggest that habitat coupling by waterbirds may be controlled by fish, which can suppress external prey subsidies, apart from being an attractive food for piscivorous birds.     

Pliocene and Pleistocene diatom datum levels related with paleoceanography in the northwest Pacific

Thick sections of Pliocene and Pleistocene biosiliceous clay and ooze were recovered by the Hydraulic Piston Corer (HPC) at three northwest Pacific sites (DSDP Sites 578, 579, and 580). They contain a well-preserved paleomagnetic record which made it possible to evaluate diatom events used in low and high latitudes in the transitional region of the northwest Pacific. Equatorial Pacific events are usually isochronous between the equatorial and subarctic regions. However, species which have short ranges in low latitudes tend to have diachronous first and last appearances in higher latitudes. All subarctic North Pacific datum species are present in the sediments at three sites which lie north and south across the subarctic front, but their ranges become shorter in southern regions. They do not penetrate into the equatorial region. Spatial distributions of these events are influenced by the paleo-position of the subarctic front. The migration of species from their home-area outwards, in the form of the first appearance, is related to the fluctuations of the subarctic front. The last appearance of species is a response to the change of the surface water temperature that is beyond the limit of tolerance of the species, or an unstable oceanic environment due to major change of climate.     

大鵟的食性改变:来自稳定性碳同位素的证据

通过对大规模灭鼠后的海北高寒草甸生态系统大鵟、小型哺乳类以及雀形目鸟类肌肉稳定性碳同位素比值的测定发现,大篱的肌肉稳定性碳同位素比值介于-22．60‰和-23．10‰之间;小型哺乳类和主要雀形目鸟类肌肉稳定性碳同位素比值分别介于-25．57‰和-25．78‰以及-24．81‰和-22．51‰之间,且它们之间差异性显著。基于碳同位素的分馏模式(即动物和它食物之间稳定性碳同位素分馏在1‰～2‰之间),我们推断经大规模灭鼠后,大篱的食性发生了较大变化,其食物主要来源于高寒草甸的雀形目鸟类,而非原来的小型哺乳类。通过稳定性同位素营养级模型的运算发现,大蔫处在4．23左右的营养级;雀形目鸟类处在2．4到3．39左右的营养级,而小型哺乳类则处在1．45到1．82左右的营养级。进而采用稳定性同位素质量平衡模型计算得出,大篱的食物由35．04％的小型哺乳类和64．96％的雀形目鸟类所组成,进一步说明小型哺乳类在大量灭鼠后仅占大篱食物的很小一部分。由此可见,采取大规模的化学灭鼠,不仅降低了小型啮齿类天敌——大鵟的数量,而日使得其食谱发毕了巨大改变而转向草甸主要雀形目鸟类[动物学报49(6)：764～768,2003]。     

Late neogene biostratigraphy and paleoceanography of DSDP site 310 Central North Pacific and correlation with the Southwest Pacific

Late Neogene planktonic foraminifera have been examined at Site 310 in the Central North Pacific and their stratigraphic ranges and frequencies are presented here. Blow's (1969) zonation developed for tropical regions has been applied where applicable. Where tropical index taxa are rare or absent in this temperate region, Globorotalia crassaformis, and the evolutionary bioseries G. conoidea — G. conomiozea and G. puncticula — G. inflata have been found useful for zonal subdivisions. A correlation between stratigraphic ranges and frequency distributions of these species at Site 310 in the Central North Pacific, and Site 284 in the Southwest Pacific indicates that these species are relatively consistent biostratigraphic markers in temperate regions of both the North and South Pacific Oceans. An informal zonation for temperate latitudes of the Southwest Pacific has been established by Kennett (1973) and a similar zonal subdivision can be made at Site 310.Paleoclimatic/paleoceanographic interpretations based on coiling ratios, percent abundance, and phenotypic variations of Neogloboquadrina pachyderma indicate four major cold events during early, middle, and late Pliocene, and early Pleistocene. Faunal correlations of these events with similar events elsewhere in the Northeast and Southwest Pacific which have been paleomagnetically dated indicate the following approximate ages for these cold events: 4.7 Ma, 3.0 Ma, 2.6–1.8 Ma. and 1.2 Ma. Faunal assemblages have been divided into three groups representing cool, intermediate, and warmer water assemblages. Cool water assemblages are dominated by >60% N. pachyderma; intermediate temperature faunas are dominated by species of Globigerina and Globigerinita and contain between 20% and 30% N. pachyderma. Warmer water assemblages are dominated by species of Globorotalia and contain <10% N. pachyderma. Frequency oscillations within these groups, in addition to paleotemperature parameters evident in N. pachyderma, afford refined paleoclimatic/paleoceanographic interpretations.     

Barnacles (Cirripedia: Thoracica) from the equatorial East Pacific

Previously unidentified barnacles (Cirripedia: Thoracica) that were collected during the ninth cruise of the R/V Akademik Mstislav Keldysh in the equatorial zone of the East Pacific Ocean from depths of 1422–1490 m are examined. The following species were found in two samples we studied: Amigdoscalpellum costellatum, A. elegans, Arcoscalpellum michelottianum, Annandaleum gruvelii gruvelii, Cristallinaverruca allisoni, and Metaverruca recta. These species are described, illustrated, and compared to the published data. This study extends the previously known areas of distribution of some species and adds further data on their morphology.     

Tarbellastraea (Scleractinia): A new stable isotope archive for Late Miocene paleoenvironments in the Mediterranean

Geochemical proxy records of sea surface temperature (SST) or sea surface salinity (SSS) variability on intra- and interannual time-scales in corals from geological periods older than Pleistocene are extremely rare due to pervasive diagenetic alteration of coralline aragonite. Very recently, however, stable isotope data (δ¹⁸O, δ¹³C) from specimens of Porites of Late Miocene age (10 Ma) have been shown to preserve original environmental signatures. In this paper we describe new finds of the zooxanthellate corals Porites and Tarbellastraea in exceptional aragonite preservation from the island of Crete in sediments of Tortonian (～ 9 Ma) and Early Messinian (～ 7 Ma) age. Systematic, comparative stable isotope analysis of massive Tarbellastraea and Porites sampled from the same beds and localities reveal identical stable isotope fractionation patterns in both genera. Therefore, extinct Tarbellastraea represents an additional environmental archive fully compatible and mutually exchangeable with Porites. Provided that seasonal variations in δ¹⁸O reflect SST changes only, seasonal SST contrasts of 7.3 °C for the Tortonian and 4.8 °C for the Early Messinian are inferred, implying warmer summer and cooler winter SSTs during the Tortonian than during the Messinian. However, reduced δ¹⁸O seasonality (1.1‰ in the Tortonian and 0.7‰ in the Messinian) and slightly less negative mean δ¹⁸O in Messinian corals (? 2.4‰) compared to Tortonian specimens (? 2.7‰) may not necessarily indicate a long-term fall in SSTs, but changes in surface water δ¹⁸O, i.e. global ice build-up or enhanced evaporation during summer or increased precipitation/river discharge during winter and changes in insolation. On the other hand, coral communities of Tortonian and Messinian age in central Crete are identical, and compatible annual extension rates indicate similar average SSTs during the two investigated time periods. In addition, lithological and paleobotanical data from Central Crete document a change from humid to dry climatic conditions during the Late Miocene. Therefore, a likely explanation for the observed shift in coral mean δ¹⁸O and reduced δ¹⁸O seasonality from the Tortonian to the Early Messinian is a change in ambient seawater δ¹⁸O caused by a change in the hydrological balance towards high evaporation/high salinity during summer.