Marker Horizons Research Articles

Synchronisation of sedimentary records using tephra: A postglacial tephrochronological model for the Chilean Lake District

Well-characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives. The use of tephra as a chronological tool is however still widely underutilised in southern Chile and Argentina. In this study we develop a postglacial tephrochronological model for the Chilean Lake District (ca. 38 to 42°S) by integrating terrestrial and lacustrine records. Tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout. By combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date. We present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental, palaeoclimatological and palaeoseismological purposes. Most volcanoes in the studied segment of the Southern Volcanic Zone, between Llaima and Calbuco, have produced at least one regional marker deposit resulting from a large explosive eruption (magnitude ≥ 4), some of which now have a significantly improved age estimate (e.g., the 10.5 ka Llaima Pumice eruption from Llaima volcano). Others, including several units from Puyehue-Cordón Caulle, are newly described here. We also find tephra related to the Cha1 eruption from Chaitén volcano in lake sediments up to 400 km north from source. Several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives. Our chronological model suggests three distinct phases of eruptive activity impacting the area, with an early-to-mid-Holocene period of relative quiescence. Extending our tephrochronological framework further south into Patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial.

Tephra layers along the southern Tyrrhenian coast of Italy: Links to the X-5 & X-6 using volcanic glass geochemistry

We present the geochemistry of glass fragments from three visible tephra layers outcropping in Southern Italy. Two tephra deposits (LeS1 and LeS2), outcropping in the Palinuro area (Cilento, Southern Italy), correspond stratigraphically to the CIL1 and CIL2 tephra units (Giaccio et al., 2012), respectively; in turn these are correlated with the X-5 (ca. 105ka) and X-6 (ca. 108ka) marine tephra markers on the basis of their major element glass compositions. In order to reinforce these tephra correlations we examine both their major and trace element glass compositions. LeS1 compositions were compared with other correlatives of the X-5 tephra layer (POP3 [Sulmona basin], TM-25 [Lago Grande di Monticchio (LGdM)], CIL1 [Palinuro]) from the central Mediterranean. Our data validate a correlation of the Palinuro tephra layer, LeS1, to the X-5 correlatives. The Palinuro tephra deposit, LeS2, has glass compositions which correspond precisely with correlatives of the X-6 marker tephra (CIL2 [Palinuro], TM-27 [LGdM], I-9 [Ionian Sea] and PRAD 2812 [PRAD1-2, Adriatic Sea]). A tephra in the Valle del Crati (Calabria) appears to overlap in composition with the LeS2 tephra, indicating a probable correlation with the X-6 marker. These new data provide a detailed geochemical characterisation of two widespread marker horizons and are crucial for establishing precise correlations of sedimentary archives across the central Mediterranean region.

Paleoseismic investigations in the Kopili Fault Zone of North East India: Evidences from liquefaction chronology

We report the seismogenic liquefaction signatures observed in the Kopili Fault Zone of the Brahmaputra plains, NE India. This seismically active zone has previously been identified as the “Assam seismic gap” and thus necessitates understanding its past seismicity and implied seismic hazard. With this objective, paleo-seismic studies using seismogenic liquefaction features have been carried out in this region largely covered with the flood plain deposits of Kopili and Kalang rivers. The trenches excavated at two locations revealed extensive liquefaction features with more than 20 sub parallel sand dykes having major orientation in NE–SW direction. A total of 29 samples from marker horizons have been processed to constrain chronology of the liquefaction features using optically stimulated luminescence (OSL) and 14C (AMS) dating techniques. The age constraints in terms of respective lower and upper bound age brackets for individual dykes suggest three time intervals of their formations i.e. (i) 250±25yr. BP, (ii) between 400 to 770yr. BP and (iii) 900±50yr. BP. These new ages of liquefaction features correspond to the occurrence timings of causative seismic events which are in addition to the known historical earthquakes and thus enhance our understanding of the paleoseismic history of this region during past ~1000years.

Upper Pleistocene loess-palaeosol records from Northern France in the European context: Environmental background and dating of the Middle Palaeolithic

In Northern France the loess cover from the Last glacial (Weichselian) is represented by a semi-continuous mantle up to 8 m in thickness in the best localities such as leeward slopes (E–NE exposures). In this large area, pedostratigraphic sequences from the last Interglacial-glacial cycle (Eemian-Weichselian) have been intensively studied, especially under the auspices of active rescue archaeological programmes that have provided hundreds of individual sequences from test-pits or excavations. In spite of variations in the thickness of the different stratigraphic units, driven by differences in geomorphological contexts, the pedostratigraphic sequences from the last Interglacial-glacial cycle exhibit a particularly constant pedosedimentary pattern, including well-identified pedological and periglacial marker horizons that can be followed towards the East in Belgium, in Germany and even in Central Europe. According to the newest data, it is shown that northern France loess-palaeosol sequences are well suited to record the response of Western European environments to rapid climatic changes (Dansgaard–Oeschger cycles). The main objective of this paper is to present a summary of the pedostratigraphic sequence from Northern France, supplemented by the data from the new reference sequence of Havrincourt and a global correlation scheme with surrounding areas. In Northern France, the synthesis of the observations carried out on ca. 100 sequences during the last 20 years allows the establishment of a highly detailed pedostratigraphic and chronostratigraphic scheme that represents a unique database for the discussion of the relations between Palaeolithic occupation and environment. In this context, a strong relationship between the intensity of human occupation and the climatic and environmental context is demonstrated. This relationship appears to be conditioned by the relative abundance of large mammal fauna, itself linked to vegetation density, as indicated by the extremely sparse biomass and the hiatus in human occupation that characterise Upper Pleniglacial loess.

Elevation dynamics in a restored versus a submerging salt marsh in Long Island Sound

Accelerated sea-level rise (SLR) poses the threat of salt marsh submergence, especially in marshes that are relatively low-lying. At the same time, restoration efforts are producing new low-lying marshes, many of which are thriving and avoiding submergence. To understand the causes of these different fates, we studied two Long Island Sound marshes: one that is experiencing submergence and mudflat expansion, and one that is undergoing successful restoration. We examined sedimentation using a variety of methods, each of which captures different time periods and different aspects of marsh elevation change: surface-elevation tables, marker horizons, sediment cores, and sediment traps. We also studied marsh hydrology, productivity, respiration, nutrient content, and suspended sediment. We found that, despite the expansion of mudflat in the submerging marsh, the areas that remain vegetated have been gaining elevation at roughly the rate of SLR over the last 10 years. However, this elevation gain was only possible thanks to an increase in belowground volume, which may be a temporary response to waterlogging. In addition, accretion rates in the first half of the twentieth century were much lower than current rates, so century-scale accretion in the submerging marsh was lower than SLR. In contrast, at the restored marsh, accretion rates are now averaging about 10 mm yr−1 (several times the rate of SLR), much higher than before restoration. The main cause of the different trajectories at the two marshes appeared to be the availability of suspended sediment, which was much higher in the restored marsh. We considered and rejected alternative hypotheses, including differences in tidal flooding, plant productivity, and nutrient loading. In the submerging marsh, suspended and deposited sediment had relatively high organic content, which may be a useful indicator of sediment starvation.

Contemporary Deposition and Long-Term Accumulation of Sediment and Nutrients by Tidal Freshwater Forested Wetlands Impacted by Sea Level Rise

Contemporary deposition (artificial marker horizon, 3.5 years) and long-term accumulation rates (210Pb profiles, ~150 years) of sediment and associated carbon (C), nitrogen (N), and phosphorus (P) were measured in wetlands along the tidal Savannah and Waccamaw rivers in the southeastern USA. Four sites along each river spanned an upstream-to-downstream salinification gradient, from upriver tidal freshwater forested wetland (TFFW), through moderately and highly salt-impacted forested wetlands, to oligohaline marsh downriver. Contemporary deposition rates (sediment, C, N, and P) were greatest in oligohaline marsh and lowest in TFFW along both rivers. Greater rates of deposition in oligohaline and salt-stressed forested wetlands were associated with a shift to greater clay and metal content that is likely associated with a change from low availability of watershed-derived sediment to TFFW and to greater availability of a coastal sediment source to oligohaline wetlands. Long-term accumulation rates along the Waccamaw River had the opposite spatial pattern compared to contemporary deposition, with greater rates in TFFW that declined to oligohaline marsh. Long-term sediment and elemental mass accumulation rates also were 3–9× lower than contemporary deposition rates. In comparison to other studies, sediment and associated nutrient accumulation in TFFW are lower than downriver/estuarine freshwater, oligohaline, and salt marshes, suggesting a reduced capacity for surface sedimentation (short-term) as well as shallow soil processes (long-term sedimentation) to offset sea level rise in TFFW. Nonetheless, their potentially large spatial extent suggests that TFFW have a large impact on the transport and fate of sediment and nutrients in tidal rivers and estuaries.

The Sokoliska Limestone, a new regional marker horizon of coccolith laminites in the Oligocene of the Outer Carpathians: diagnostic features and stratigraphic position

Isochronous horizons of pelagic coccolith limestones are important regional stratigraphic markers in the Oligocene sequences of the Outer Carpathians. Three widely recognized limestone horizons are the Tylawa Limestones, the Jaslo Limestone and the Zagorz Limestone horizons. Another regional marker horizon is described here, the Sokoliska Limestone horizon, situated between the Jaslo Limestone and Zagorz Limestone horizons in the nannoplankton NP24 Zone. Its lithologic characteristics allow it to be distinguished from the other limestones, especially under the optical microscope. Its dark laminae are greatly enriched in non-calcareous material. The limestone contains tests of planktonic foraminifers, but these are much less abundant than in the Jaslo Limestone. The boundaries with the marly shales above and below are often more gradational than is the case with the other limestone horizons. The Sokoliska Limestone has been recognized over a distance of ca. 550 km in the Skole (Tarcau) and Silesian units of the eastern and northern parts of the Outer Carpathians. Four sections of the Sokoliska Limestone horizon in the Polish and Romanian Carpathians are described; the exposure in the Sokoliska cliff of the Solinka River at Bukowiec was selected as the reference locality.

The Sokoliska Limestone, a new regional marker horizon of coccolith laminites in the Oligocene of the Outer Carpathians: diagnostic features and stratigraphic position

The Sokoliska Limestone, a new regional marker horizon of coccolith laminites in the Oligocene of the Outer Carpathians: diagnostic features and stratigraphic position

Balanced Sediment Fluxes in Southern California’s Mediterranean-Climate Zone Salt Marshes

Salt marsh elevation and geomorphic stability depends on mineral sedimentation. Many Mediterranean-climate salt marshes along southern California, USA coast import sediment during El Nino storm events, but sediment fluxes and mechanisms during dry weather are potentially important for marsh stability. We calculated tidal creek sediment fluxes within a highly modified, sediment-starved, 1.5-km2 salt marsh (Seal Beach) and a less modified 1-km2 marsh (Mugu) with fluvial sediment supply. We measured salt marsh plain suspended sediment concentration and vertical accretion using single stage samplers and marker horizons. At Seal Beach, a 2014 storm yielded 39 and 28 g/s mean sediment fluxes and imported 12,000 and 8800 kg in a western and eastern channel. Western channel storm imports offset 8700 kg exported during 2 months of dry weather, while eastern channel storm imports augmented 9200 kg imported during dry weather. During the storm at Mugu, suspended sediment concentrations on the marsh plain increased by a factor of four; accretion was 1–2 mm near creek levees. An exceptionally high tide sequence yielded 4.4 g/s mean sediment flux, importing 1700 kg: 20 % of Mugu’s dry weather fluxes. Overall, low sediment fluxes were observed, suggesting that these salt marshes are geomorphically stable during dry weather conditions. Results suggest storms and high lunar tides may play large roles, importing sediment and maintaining dry weather sediment flux balances for southern California salt marshes. However, under future climate change and sea level rise scenarios, results suggest that balanced sediment fluxes lead to marsh elevational instability based on estimated mineral sediment deficits.

Structure and Stratigraphy from Aeromagnetic Data within Sedimentary Basins Minerals keynote paper

Sedimentary rocks commonly contain sufficient magnetic mineral to yield coherent signal in modern aeromagnetic surveys. Careful acquisition procedures and astute application of conventional processing methodology allow the signal from the sedimentary section to be recognised and isolated from that of 'basement', facilitating focused interpretations of each these geological domains. Spectral separation filtering underpins this process, providing imagery of magnetic sources that lie at differing depths. The integration and interpretation of the resulting 'spectral depth windows' follows the steps used to integrate aeromagnetics with geology in 'hard-rock' domains. A substantial phase of basic observations on the aeromagnetic imagery is the fundamental first step, and commitment to the integration of the best available geology to yield lithostratigraphic and structural framework interpretations completes the initial task. Forward and inverse modelling constrains the qualitative geological interpretation, and enables hypotheses formed during that interpretation to be tested. The two examples presented show the virtues of separation filtering and the style of geological interpretation that can be derived using the aeromagnetic data as the driver. The Amadeus Basin example presents a 'feast' of shallow magnetic rock units, many of which are probably magnetic stratigraphic marker horizons. These yield a thought-provoking interpretation of structure and stratigraphy in the upper sedimentary section. The discordance between the geometries of the shallow and deep magnetic rock units raises the likelihood of a major detachment structure between the two geological domains. The aeromagnetic data from the second example, in the Galmoy Pb-Zb district in Ireland, presents major challenges. Separation filtering yields a set of very low amplitude but strongly coherent spectral depth windows which, when interpreted, provide a range of structural and stratigraphic features that relate directly to the mineralised environment. The successful application of aeromagnetics in this erstwhile 'non-magnetic' geological domain is testimony to the value of the technique in most, if not all sedimentary basins.

Visualizing tephra deposits and sedimentary processes in the marine environment: The potential of X-ray microtomography.

Localized tephra deposition in marine sequences is the product of many complex primary and secondary depositional processes. These can significantly influence the potential applicability of tephra deposits as isochronous marker horizons and current techniques, used in isolation, may be insufficient to fully unravel these processes. Here we demonstrate the innovative application of X‐ray microtomography (µCT) to successfully identify tephra deposits preserved within marine sediments and use these parameters to reconstruct their internal three‐dimensional structure. Three‐dimensional visualizations and animations of tephra dispersal in the sediment permit a more thorough assessment of postdepositional processes revealing a number of complex microsedimentological features that are not revealed by conventional methods. These features include bioturbation burrows and horizontally discontinuous tephra packages, which have important ramifications for the stratigraphic placement of the isochron in a sedimentary sequence. Our results demonstrate the potential for utilizing rigorous two and three‐dimensional microsedimentological analysis of the ichnofabric to enhance and support the use of tephra deposits as isochronous marker horizons and to identify the stratigraphic position that best reflects the primary fallout of ash. The application also provides an exceptional insight into the style and rate of sedimentation processes and permits an assessment of the stratigraphic integrity of a tephra deposit. We discuss the possibility of applying these µCT methods to the identification of cryptotephras within various paleoclimatic sequences and to enhance our understanding of marine sedimentation processes.

Deciphering the Hydrothermal Evolution of a VMS System by LA-ICP-MS Using Trace Elements in Pyrite: An Example from the Bracemac-McLeod Deposits, Abitibi, Canada, and Implications for Exploration

Textural and chemical characterization of pyrite was used to reconstruct the hydrothermal evolution of the Bracemac-McLeod Archean volcanogenic massive sulfide (VMS) deposits (~6 Mt) in the Matagami district, Abitibi, Canada. The mineralization, hosted in a bimodal volcanic sequence, is divided into a (1) Zn-rich zone, concordant to the Key Tuffite— a marker horizon at the district-scale, 2) a Cu-rich zone which locally crosscuts the stratigraphic pile, and 3) some localized magnetite-rich zones replacing sphalerite and pyrite. Detailed petrographic and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) studies of whole pyrite grains (including inclusions and zonation) from the Key Tuffite and from the different ore zones of the deposits, has revealed five pyrite types. Nodular pyrites (I) in the Key Tuffite and subidiomorphic pyrites from the low temperature (250°C) Zn-rich zone (II) have the same chemical signature, suggesting that both precipitated under similar physicochemical conditions. A wide range of trace elements (Cu, Zn, Ag, Sn, Sb, Te, Au, Pb, Bi, Tl) are present in both, but compared to the other types of pyrite, they are significantly enriched in Sb and Tl. Subidiomorphic pyrites from the copper-rich zone (III) have a different signature, enriched only in Se ± Co, In, Pb, and Bi. These pyrites commonly overgrow the nodular pyrites and are related to later higher-temperature fluids (300°C). Pyrite preserved during a later magnetite-replacement (>350°C) stage (IV) also are enriched in Se, but are depleted in all other elements. A similar signature is found in idiomorphic metamorphic pyrite (V), which occurs at the district scale. During high temperature recrystallization of pyrite IV and V, only the contents of Ni, Co, As, and Se are mostly preserved, whereas other base metals are expelled from the pyrite structure. Consequently, only Ni, Co, As, Se, Sb, and Tl were useful to reconstruct the hydrothermal evolution of the VMS system and used to build a suite of discrimination diagrams. Specifically, the ratio Se/Tl can discriminate pyrites from mineralized Zn-rich zone (Se/Tl 10,000). The innovative approach of this study, based on LA-ICP-MS acquisition of the chemistry of entire pyrite grains, makes our results directly applicable for exploration. Consequently, pyrite concentrate samples, analyzed for As, Tl, and Se by more classical mass-spectrometric techniques, have the potential to be used for vectoring to ore by using the proposed discrimination diagrams.

Mangrove Sedimentation and Response to Relative Sea-Level Rise

Mangroves occur on upper intertidal shorelines in the tropics and subtropics. Complex hydrodynamic and salinity conditions, related primarily to elevation and hydroperiod, influence mangrove distributions; this review considers how these distributions change over time. Accumulation rates of allochthonous and autochthonous sediment, both inorganic and organic, vary between and within different settings. Abundant terrigenous sediment can form dynamic mudbanks, and tides redistribute sediment, contrasting with mangrove peat in sediment-starved carbonate settings. Sediments underlying mangroves sequester carbon but also contain paleoenvironmental records of adjustments to past sea-level changes. Radiometric dating indicates long-term sedimentation, whereas measurements made using surface elevation tables and marker horizons provide shorter perspectives, indicating shallow subsurface processes of root growth and substrate autocompaction. Many tropical deltas also experience deep subsidence, which augments relative sea-level rise. The persistence of mangroves implies an ability to cope with moderately high rates of relative sea-level rise. However, many human pressures threaten mangroves, resulting in a continuing decline in their extent throughout the tropics.

Distribution, mineralogy and geochemistry of silica-iron exhalites and related rocks from the Tyrone Igneous Complex: Implications for VMS mineralization in Northern Ireland

Iron formations, hematitic cherts (jaspers), ‘tuffites’, silica-iron exhalites and other metalliferous chemical sedimentary rocks are important stratigraphic marker horizons in a number of volcanogenic massive sulfide (VMS) districts worldwide, forming during episodes of regional hydrothermal activity. The VMS prospective ca. 484–464Ma Tyrone Igneous Complex of Northern Ireland represents a structurally dissected arc-ophiolite complex that was accreted to the composite margin of Laurentia during the Grampian orogeny (ca. 475–465Ma), and a potential broad correlative to the VMS-rich Buchans–Robert's Arm arc system of the Newfoundland Appalachians. Silica-iron-rich rocks occur at several stratigraphic levels in the Tyrone Igneous Complex spatially and temporally associated with rift-related basalts (e.g., Fe–Ti-rich eMORB, IAT, OIB) and zones of locally intense hydrothermal alteration. In the ca. 475–474Ma lower Tyrone Volcanic Group, these rocks are characterized by massive, 1–5m thick blood-red jaspers, hematitic siltstones and mudstones, and intensely silica-hematite altered tuffs and flows. Their mineralogy is dominated by quartz–hematite±magnetite–(chlorite-sericite±tremolite/actinolite), with Fe concentrations rarely exceeding 10wt.%. Relict textures (including the presence of coalesced spherules of silica-iron oxides) in rocks exposed at Tanderagee NW, Creggan Lough and Tory's Hole are indicative of seafloor exhalation, whereas replacement of the original volcanic stratigraphy is evident to varying degrees at Tanderagee, Beaghbeg and Bonnety Bush. In the structurally overlying ca. 473–469Ma upper Tyrone Volcanic Group, chemical sedimentary rocks include recrystallized: (i) thin and laterally-restricted jaspers in thick sequences of graphitic pelite at Boheragh; and (ii) laterally-persistent sulfidic cherts and ironstones dominated by quartz–hematite–magnetite–(chlorite) or quartz–pyrite–(chlorite) in sequences of tuff at Broughderg. Compared to chemical sedimentary rocks associated with VMS deposits worldwide, their geochemical characteristics are most similar to silica-iron exhalites of the Mount Windsor Subprovince (SE Australia) and jaspers of Central Arizona, Bald Mountain (Northern Maine), the Urals, Iberian Pyrite Belt and Løkken ophiolite (Norway). Positive Eu anomalies (at Slieve Gallion and Tanderagee NW), elevated Cu+Pb+Zn, Au, Fe/Ti, Fe/Mn, Sb, Ba/Zr and Fe+Mn/Al, together with low REE, Sc, Zr and Th, are indicative of a greater hydrothermal component and potentially more VMS-proximal signatures. Based on bulk ironstone geochemistry, Bonnety Bush, Tanderagee NW-Creggan Lough, Broughderg and Drummuck (Slieve Gallion) are considered the most VMS prospective areas in the Tyrone Igneous Complex and warrant further exploration.

New data on the development of the Dunbarella marine marker horizon in the Lublin Coal Basin (Poland)

Dunbarella marine marker horizon has significant stratigraphical importance for the Lublin Coal Basin — LCB (Poland). It sets the boundary between Westphalian A (Langsettian) and B (Duckmantian) and between paralic and limnic sedimentation at the same time. This marker horizon is also the base of the Lublin formation deposits where a main multi-seam coal deposits can be found. The horizon correlates with the Clay Cross Marine Band (England), the Katharina horizon (Germany, The Netherlands) and the Quaregnon horizon (Belgium). The paleontological material taken from recent boreholes provides new data about development of the Dunbarella horizon of the LCB.32 bivalve species have been encountered and can be assigned to 28 genera. Edmondia, Posidonia, Sanguinolites, Polidevcia, Phestia and Dunbarella are the most abundant in all profiles. Slightly less numerous are gastropods (9 genera), represented mainly by Glabrocingulum and Euphemites, and cephalopods (4 genera), represented mainly by Gastrioceras and Anthracoceras. Very frequent, although not so diverse, are the brachiopods, primarily inarticulate and represented almost exclusively by Lingula. Studies of macrofauna showed the presence of marker species and faunal assemblages typical of the Dunbarella horizon. A distinctive feature of this horizon which was observed was the cyclic sedimentation and associated variability of the fauna spectrum. This is typical for Westphalian marine horizons of Northwestern Europe, including those correlated with the Dunbarella horizon.

Porphyry Au-Cu mineralization controlled by reactivation of an arc-transverse volcanosedimentary subbasin

Located in the accreted remnants of the Ordovician Macquarie Arc (Australia), Cadia East is the largest alkalic porphyry Au-Cu deposit currently known. The deposit is centered on a 2-km-long, northwest-trending volcanosedimentary subbasin, which was identified by mapping variations in the distribution and thickness of faulted sedimentary marker horizons. The normal faults that define the subbasin predate porphyry mineralization and are oriented parallel to a major arc-transverse lineament. Porphyry Au-Cu mineralization was controlled by reactivation of the subbasin during postorogenic extension as the Macquarie Arc was accreted to the Gondwana margin in the early Silurian. Dilation of the northwest-trending subbasin faults facilitated emplacement of alkalic porphyry dikes and associated sheeted quartz-sulfide veins that define the Cadia East orebody. During the middle Silurian, north-trending fault-bound marine basins buried Cadia East, significantly enhancing preservation of the ore system. These north-trending faults were optimally oriented for reactivation during Devonian east-directed compression, leaving the mineralized Cadia East volcanosedimentary subbasin largely intact. We identify the significance of reactivated premineralization structures within an arc-transverse lineament in controlling porphyry Au-Cu mineralization in a postsubduction setting. At Cadia East, the manifestation of such premineralization structures can be observed at regional, district, and deposit scales, and highlights the influence of preexisting crustal architecture on the structural character, geometry, and preservation potential of porphyry deposits throughout the arc life cycle.

The onset of the palaeoanthropocene in Iceland: Changes in complex natural systems

Pre-industrial human impacts on the past environment are apparent in different proxy records at different times in different places. Recognizing environmentally transformative human impacts in palaeoenvironmental archives, as opposed to natural variability, is a key challenge in understanding the nature of the transition to the Earth’s current ‘Anthropocene’ condition. Here, we consider the palaeoenvironmental record for Iceland over the past 2.5 ka, both before and after the late ninth century human settlement ( landnám). The Scandinavian colonization of the island was essentially abrupt, involving thousands of people over a short period. The colonization triggered extensive changes in Icelandic ecosystems and landscapes. A volcanic ash known as the Landnám tephra was deposited over most of Iceland immediately before the settlement began. The Landnám tephra layer thus provides a uniquely precise litho-chrono-stratigraphic marker of colonization. We utilize this marker horizon as an independent definition of the effective onset of the local palaeoanthropocene (which is conceptually related to, but distinct from, the global Anthropocene). This allows us to evaluate proxy records for human impact on the Icelandic environment and to assess how and when they show transformative impact. Based on this analysis, we consider the implications for understanding and defining the Anthropocene in those areas of the Earth where such a clear independent marker of the onset of significant human impacts is lacking.

Distal tephrochronology in volcanic regions: Challenges and insights from Kamchatkan lake sediments

Kamchatka is one of the world's most active volcanic regions and has hosted many explosive eruptions during the Holocene. These eruptions had the potential to disperse tephra over wide areas, forming time-synchronous markers wherever those tephras are found. Recent research in Kamchatka has begun to focus on the geochemical analysis of individual glass shards in order to characterise tephra layers. We have applied this approach to the study of visible tephras from three lakes – one in central and two in northern Kamchatka – with the aim of identifying key tephras and potential issues in the application of distal (>100km from an active volcano) tephra in volcanically complex regions. In total, 23 tephras from 22 tephra beds have been geochemically analysed, representing products from at least four volcanic systems in Kamchatka. We demonstrate that distal lake sediments in the region can yield reliable tephrostratigraphies, capturing tephra from eruptions that have the greatest potential to disperse volcanic ash beyond the region. We draw attention to issues relating to correlating and distinguishing key marker horizons from the highly active Shiveluch Volcano, namely the need to ensure inter-lab comparability of geochemical data and good chronological control of the proximal and distal tephras. Importantly, we have also extended the known distribution of two key tephra isochrons from the Ksudach volcano. Our work contributes valuable glass geochemical on data several key marker beds that will facilitate future tephra and palaeoenvironmental research within and beyond Kamchatka.

Interactions between Late Quaternary volcanic and sedimentary processes in the Naples Bay, Southern Tyrrhenian sea

The interactions between Late Quaternary volcanic and sedimentary processes in the Naples Bay, Southern Tyrrhenian sea, are here discussed through the results of the marine geological survey at the scale 1:25.000. The example of the geological map n. 465 “Isola di Procida”, herein presented, has put in evidence the stratigraphy of marine Quaternary deposits and related volcanic seismic units. The volcanic deposits cropping out in the Procida island have been explained as the result of eruptions of local eruptive centres. The geological survey carried out onshore indicates the occurrence of several pyroclastic units linked to the eruptive activity of the Ischian and Phlegrean volcanic complexes, interstratified with the products erupted by local volcanic centres. The occurrence in the pyroclastic sequences of Ischia and Procida islands of several marker horizons and their stratigraphic correlations have allowed to reconstruct the volcanological evolution of the two islands and the interactions with sedimentary processes at the scale of the whole Tyrrhenian margin. Four geological maps at the scale 1:25.000 covering the whole Naples Bay have been reconstructed based on the interpretation of marine geological and geophysical data. The stratigraphic relationships between the seismic units and the eruptive deposits have testified the activity of several monogenetic volcanic centers, whose products are interstratified with marine and continental deposits of the Late Quaternary depositional sequence.

Evidence of Late Holocene shoreline progradation in the coast of Kerala, South India obtained from OSL dating of palaeo-beach ridges

The luminescence chronology of beach ridges in the central part of Kerala along the west coast of India has been made for the first time in order to reconstruct past sea-level highstands and coastline progradation. Transitional boundaries between shoreface to foreshore and foreshore to dune (aeolian) environments were used as marker horizons for determining the palaeo-shoreline as well as sea-level indicators. The study presents the results of optically-stimulated luminescence (OSL) dating of sand samples collected from wave- and wind-built horizons derived from quartz using single aliquot regenerative dose (SAR) protocol. The OSL ages from most of the sites are consistent with each other, indicating a stage of coastline progradation between 3 and 5ka and widespread beach ridge formation all along the west coast of Kerala during this period. The OSL ages of transition levels together with the elevation measured using differential global positioning system (DGPS) indicates that the sea level was between 3.5 and 4.5m above present MSL during the Late Holocene period. It may also imply the emergence of the coast due to neo-tectonic activity as the sea-level curve for the east coast from reported ages indicates a mean elevation between 1.5 and 2.5m. The ages are discussed in the light of reported events, such as sea-level changes, coastal geomorphology, hydrological variations and neo-tectonics.