Periods Of Rapid Sea-level Rise Research Articles

MICROBIALITE OCCURRENCE AND PATTERNS IN HOLOCENE REEFS OF BORA BORA, SOCIETY ISLANDS

ABSTRACT The thickness of microbialite crusts in Holocene barrier and fringing reefs of Bora Bora was quantified in drill cores from windward and leeward settings to decipher possible spatial and temporal patterns as well as controlling environmental factors. Based on the analysis of 145 occurrences in nine rotary cores, microbialite thickness ranges from 0.1–11.0 cm with an average value of 1.97 cm (SD = 2.47). Microbialites occur only from 9.5–5.6 ka corresponding to a period of rapid sea-level rise and reef accretion in the early Holocene. However, there is no statistically significant correlation between microbialite thickness and reef accretion rate. Also, there is no correlation between microbialite abundance and age. The upcore increase in microbialite abundance, however, suggests that time available for carbonate accretion in shallow water plays a role in microbialite formation. Crust thickness is greater on windward as compared to leeward fringing reef settings indicating that flushing of pore space is a likely factor controlling microbialite accretion. Other environmental factors potentially being responsible for the Holocene decrease in microbialite abundance include climate, i.e., decreasing temperatures and precipitation (supporting nutrient input by runoff) as well as decreasing seawater alkalinity. At the mesoscale, structureless and laminated microbialites are by far the most common types. Coated debris, boring infill, and digitate types are less common. Textures at the microscale, including laminated, clotted, and peloidal, do not necessarily match mesoscale textures. The Bora Bora microbialites consist in more or less equal parts of high-magnesium calcite and aragonite. The δ13C values range from +3.0 to +4.1‰ and the δ18O from -0.8 to +0.1‰. The contents of easy soluble sulfate (ESS) and carbonate associated sulfate (CAS) are relatively high. The δ18OCAS (+11.0 to +12.7‰) and δ34SCAS values (+21.9 to +23.6‰) exceed the seawater sulfate standard NBS-127 value and are in the same range as observed in other cryptic, Holocene reefal microbialites. The Bora Bora microbialites contain lipid biomarkers derived from sulfate-reducing bacteria (2–8 wt%), marine plankton, land plants, and unspecified bacteria. The former include branched, short-chain fatty acids and terminally branched fatty acids, whereby iso-fatty acids are more abundant than anteiso-fatty acids. Other compounds with terminally branched alkyl chains include iso- and anteiso-C15 and -C17 alcohols, which are interpreted as degradation products of monoalkyl glycerol ethers (MAGE). Collectively, the organic and inorganic geochemical data together with the sedimentological and mineralogical data suggest that sulfate-reducing bacteria were involved in microbialite development. The temporal and spatial distribution patterns suggest that factors such as exposure to waves and currents, time, nutrient availability, alkalinity, and climate play important roles, however, more quantitative data from other occurrences are needed to be able to better discriminate among them.

Relict and contemporary influences on the postglacial geomorphology and evolution of a current swept shelf: The Eastern Cape Coast, South Africa

Few stratigraphic models of continental shelves incorporate the process of geostrophic current-sweeping, consequently its role in the stratigraphic record is often overlooked. We examine the narrow, current-swept Eastern Cape shelf of South Africa using a combination of geophysical techniques, seafloor sampling and video observations and interpret the role of current action on the transgressive stratigraphy of this steep subtropical shelf. During the Last Glacial Maximum, fluvial valleys incised the acoustic basement rocks. During the subsequent transgression, two distinct shorelines were formed and preserved at −105 m and − 60 m. Their development and preservation is linked to (i) high sediment supply from adjacent fluvial sources, (ii) early diagenesis and (iii) alternating sea-level stillstands and periods of rapid sea-level rise during melt water pulses 1A and 1B, respectively. The deeper shoreline formed in a sandy, wide coastal plain setting with limited bedrock influence, whereas the shallower shoreline comprised alternating rock headlands and embayments like the contemporary coast. Differences in antecedent topography and geology are responsible for the temporal variability in shoreline type; coastal squeeze is exacerbated with increasing bedrock control as the shoreline migrates landwardBetween the two shoreline complexes, in the mid-shelf, the transgressive stratigraphy records initial valley infill by progradation of coast-parallel sandy spits. These are capped by a stiff lagoonal mud deposited as ongoing sea-level rise overspilled the valley interfluves, onlapping the adjacent aeolianites. The uppermost stratigraphy comprises mounds of rhodoliths which interfinger with a sandy inner to middle shelf highstand wedge.After sea-level reached its present position ca 7.4 ka yr BP, the shelf became subject to reworking by the high-energy geostrophic Agulhas Current. This has had the following major effects on the shelf stratigraphy: 1. The topographic relief of the cemented palaeo-shorelines has been emphasised by removal of the post-transgressive cover; and 2. The shelf no longer acts as a depocenter; instead, the seabed consists of rhodoliths, gravel streamers, bedrock or gravel hash of the wave ravinement surface. Given the necessary antecedent conditions such as accommodation, sediment supply and favourable diagenetic climate, prominent shorelines can form and be preserved on the shelf. Strong current sweeping emphasises these morphological features on subtropical shelves.

Geological development of the Limpopo Shelf (southern Mozambique) during the last sealevel cycle

Paleo-shorelines on continental shelves give insights into the complex development of coastlines during sealevel cycles. This study investigates the geologic development of the Limpopo Shelf during the last sealevel cycle using multichannel seismic and acoustic datasets acquired on the shelf in front of the Limpopo River mouth. A detailed investigation of seismic facies, shelf bathymetry, and a correlation to sea level revealed the presence of numerous submerged shorelines on the shelf. These shorelines are characterized by distinct topographic ridges and are interpreted as coastal dune ridges that formed in periods of intermittent sealevel still-/slowstand during transgression. The shorelines are preserved due to periods of rapid sealevel rise (melt water pulses) that led to the overstepping of the dune ridges as well as due to early cementation of accumulated sediments that increased the erosive resistance of the ridges. The high along-shelf variability of the submerged dune ridges is interpreted as a result of pre-existing topography affecting shoreline positions during transgression. The pre-existing topography is controlled by the underlying sedimentary deposits that are linked to varying fluvial sediment input at different points on the shelf. The numerous prominent submerged dune ridges form barriers for the modern fluvial sediment from the Limpopo River and dam sediment on the inner shelf. They may also facilitate along-shelf current-induced sediment transport.

On the origin of both a recent and submerged tombolo on Prvić Island in the Kvarner area (Adriatic Sea, Croatia)

This paper analyzes the origins of two tombolos (one recent and another fossil/submerged) on the southwestern coast of Prvic Island, which is located in the Kvarner area in the northeastern part of the Adriatic Sea. A recent tombolo on Cape Pipa was formed by the erosion of Quaternary deposits and Palaeogene siliciclastic rocks. The submerged tombolo is much bigger, clearly visible on the sea bed toward the southwest. The conditions for forming a submerged and recent tombolo have occurred during a slow rise and then stagnation of sea level of the Adriatic Sea in the Holocene. The sea flooded the fossil tombolo probably in the final part of a period of rapid sea-level rise at the beginning of the Holocene when a large proportion of the Quaternary sediments were eroded. Waves from the northwest (tramuntana) and southeast (jugo) refract and diffract around the tombolo. The nourishment of the beach body happens permanently on both sides of the cape. Accumulated sediments are protected by resistant rocky blocks of breccia on the peak of Cape Pipa, acting as a natural tombolo. Due to the fact that wave directions are perpendicular to the beach coastline, they do not generate longshore currents that would erode sediment in beach bodies around Cape Pipa. Therefore, the recent tombolo is probably stable in the present climatic and oceanographic conditions in the Kvarner area.

Topographic and hydrodynamic controls on barrier retreat and preservation: An example from Dogger Bank, North Sea

Barrier retreat can occur due to in-place drowning, overstepping or rollover, depending on the interplay of controls such as sea-level rise, sediment supply, coastal hydrodynamic regime and topography. Offshore sedimentary archives of barriers active during rapid Holocene sea-level rise provide important records of marine transgression, which are vital analogues to support appropriate mitigation strategies for future coastal realignment under projected relative sea-level rise scenarios. This study analyses the sedimentary archive at Dogger Bank, which is a formerly-glaciated area in the North Sea. Dogger Bank experienced marine transgression due to Early Holocene rapid relative sea-level rise. An integrated dataset of vibrocores and high-resolution seismic reflection data permits a stratigraphic framework to be established, which reveals the buried coastal geomorphology of the southern Dogger Bank for the first time. A transgressive stratigraphy was identified, comprising a topographically complicated basal glacial and terrestrial succession, overlain by two phases of barrier and tidal mudflat deposition, prior to shallow marine sedimentation. Barrier phase A was a recurved barrier drowned in place, and discontinuously overstepped to barrier phase B, which experienced continuous overstepping. By linking barrier elevations to relative sea-level curves, the timing of each barrier phase was established. Both barrier phases retreated during periods of rapid sea-level rise with abundant sediment supply. Coastal hydrodynamics (increasing wave energy) and antecedent topography with spatially variable accommodation are suggested to be the main reason for differing retreat mechanisms, rather than the rate of sea-level rise. Antecedent coastal geomorphology plays a critical role in erosional and depositional patterns during transgression, and therefore on the timing, rate and location of marine inundation, which needs to be included in models that aim to forecast hazards in coastal areas.

A below-the-present late Holocene relative sea level and the glacial isostatic adjustment during the Holocene in the Malay Peninsula

This study has reconstructed the Holocene relative sea-level history in the relatively tectonically stable Malay Peninsula with two specific objectives: quantifying the sea-level indicative meanings of mangrove sediments and producing new high-quality sea-level index points. In this study, a wetland on the east coast of the Peninsula was extensively surveyed; the elevational relationship between habitat zones and their pollen and diatom assemblages was established; and a number of indicative meanings were precisely quantified for this wetland. This quantified relationship is extrapolated to other coastal sites of the Peninsula for re-calibration of previously published mangrove-based sea-level data. The reconstructed sea-level history from the new and re-calibrated data demonstrates a period of rapid sea-level rise at a rate of 15.8 ± 3.9 mm/a between 10,500 cal a BP at −35.0 ± 4.3 m and 9000 cal a BP at −14.2 ± 1.6 m. The rate of rising sea level was gradually reduced to zero from 9000 to 5000 cal a BP, and by the end of this period the relative sea level reached a highstand of c. 3.3 ± 0.2 m. Over the past 5000 years relative sea level fell to its lowest point at −0.6 ± 0.1 m by 800 cal a BP. This far-field sea-level history has been driven by the global ice-volume change and glacial isostatic adjustment, and it matches well with the modelled ice-volume equivalent sea level.

Holocene hydrological and sea surface temperature changes in the northern coast of the South China Sea

In order to reconstruct the Holocene environmental history of a coastal site in the northern South China Sea, this study analysed the organic carbon isotope ratios (δ13Corg) and alkenone unsaturation ratios (UK′37) from a 36.5m-long sediment core drilled at seabed in the mouth region of the Pearl River estuary and generated a coupled hydrological and temperature record. This record reveals changes of monsoon-induced sediment discharge and sea surface temperature of the Holocene in four stages. In Stage I, the site was under fluvial conditions prior to postglacial marine transgression. Stage II saw an increase of sea surface temperature from c. 23.0°C to 27.0°C, associated with a strengthened summer monsoon from c. 10,350 to 8900cal. years BP. This was also a period of rapid sea-level rise and marine transgression, during which the sea inundated the palaeo-incised channel, i.e. the lower part of the T-shape accommodation space created by the rising sea. In these 1500years, fluvial discharge was strong and concentrated within the channel, and the high sedimentation rate (11.8mm/year) was very close to the rate of sea-level rise. In the subsequent 2000years (Stage III) sea level continued to rise and the sea flooded the broad seabed above the palaeo-incised channel, resulted in fluvial discharge spreading thinly across the wide accommodation space and a much reduced sedimentation rate (1.8mm/year). Sea surface temperature in this stage reached 27.3°C initially, but dropped sharply to 26.1°C towards c. 8200cal. years BP. The final stage covers the last 7000years, and the site was under a stable sea level. Sedimentation in this stage varied a little, but averaged at 1.8mm/year. Whilst fluvial discharge and sea surface temperature didn’t change much, two short periods of hydrological and temperature change were observed, which are related to the climatic cooling events of c. 4200cal. years ago and the Little Ice Age.

Geochemical and palynological sea-level proxies in hemipelagic sediments: A critical assessment from the Upper Cretaceous of the Czech Republic

Geochemical and palynological records are presented for an expanded Turonian–Coniacian hemipelagic succession in the central Bohemian Cretaceous Basin. A high-resolution stratigraphic framework is provided by biostratigraphy and organic carbon stable-isotope (δ13Corg) chemostratigraphy. A short-term (100 kyr) sea-level curve has been derived from high-resolution transgressive/regressive maxima / shore-proximity data established from basin-wide sediment geometries. The viability of geochemical and palynological parameters as potential sea-level proxies is tested against this independently derived sea-level record. Elemental chemostratigraphy is demonstrated to offer a reliable means of identifying medium- to long-term (0.4–2.4 Myr) sea-level trends. Manganese maxima are associated with periods of high sea level, and troughs with intervals of low sea level. Falling Mn contents accompany regression and rising values transgression. Major transgressive events associated with medium-term sea-level change are marked by sharp increases in Ti/Al ratios, but short-term (100 kyr) sea-level cycles are not consistently identified. Long-term δ13Corg variation and dinoflagellate cyst species richness are positively correlated and show similarities to the sea-level curve. Baseline trends have a cycle duration close to the 2.4 Myr long-eccentricity cycle. Dinocyst species richness closely follows short-term changes in sea level, with marked increases in dinocyst diversity coincident with most short-term flooding events. Periods of rapid sea-level rise caused an influx of a more diverse ‘outer shelf’ assemblage into the study area, together with the addition of shallower water species, some of which may have been transported into the central basin by hypopycnal flows. Changes in the proportion and abundance of peridinioid dinoflagellate cysts (principally Palaeohystrichophora infusorioides) were controlled principally by changing nutrient levels. Proximity proxies derived from geochemical and palynological data are not always consistent with the independent sea-level model. This exemplifies the need to understand all factors influencing elemental geochemical and palynological proxies before making simplistic sea level interpretations.

Palaeoecology, taphonomy, and preservation of a lower Pliocene shell bed (coquina) from a volcanic oceanic island (Santa Maria Island, Azores)

Abstract Massive fossil shell accumulations require particular conditions to be formed and may provide valuable insights into the sedimentary environments favouring such concentrations. Shallow-water shell beds appear to be particularly rare on reefless volcanic oceanic islands on account of narrow, steep and highly-energetic insular shelves where the potential for preservation is limited. The occurrence of an exceptional coquina (Pedra-que-pica) within the Miocene–Pliocene deposits of Santa Maria Island (Azores), therefore provides a rare opportunity to understand the conditions that led to the formation and preservation of a massive shell bed at mid-ocean insular setting. This study provides a detailed analysis regarding a 10–11-m-thick bivalve-dominated fossil assemblage exposed at Pedra-que-pica on Santa Maria Island in the Azores. Integration of taphonomical, palaeoecological and sedimentological observations are used to reconstruct the genesis of the coquina bed and related events, and to discuss why such exceptional sedimentary bodies are so rare on shelves around reefless volcanic oceanic islands. The sequence at Pedra-que-pica demonstrates a complex succession of sedimentary environments in response to the drowning of an existing coastline during a period of rapid sea-level rise. The Pedra-que-pica shell bed incorporates storm-related materials and possible debris falls that originated nearby in a shallow and highly productive carbonate factory. Deposition took place below fair-weather wave base, at around 50 m depth, as inferred from the overlying volcanic succession. The preservation of this coquina was favoured by deposition on a platform laterally protected by a rocky spur, combined with rapid burial by water-settled volcanic tuffs and subsequent volcanic effusive sequences. The recent exhumation of the deposit is the result of island uplift and subsequent erosion.

Variability in transport of terrigenous material on the shelves and the deep Arctic Ocean during the Holocene

Arctic coastal zones serve as a sensitive filter for terrigenous matter input onto the shelves via river discharge and coastal erosion. This material is further distributed across the Arctic by ocean currents and sea ice. The coastal regions are particularly vulnerable to changes related to recent climate change. We compiled a pan-Arctic review that looks into the changing Holocene sources, transport processes and sinks of terrigenous sediment in the Arctic Ocean. Existing palaeoceanographic studies demonstrate how climate warming and the disappearance of ice sheets during the early Holocene initiated eustatic sea-level rise that greatly modified the physiography of the Arctic Ocean. Sedimentation rates over the shelves and slopes were much greater during periods of rapid sea-level rise in the early and middle Holocene, as a result of the relative distance to the terrestrial sediment sources. However, estimates of suspended sediment delivery through major Arctic rivers do not indicate enhanced delivery during this time, which suggests enhanced rates of coastal erosion. The increased supply of terrigenous material to the outer shelves and deep Arctic Ocean in the early and middle Holocene might serve as analogous to forecast changes in the future Arctic.Keywords: Arctic; riverine input; coastal erosion; land–ocean interaction; Holocene.(Published: 9 December 2015)To access the supplementary material for this article, please see supplementary files in the column to the right (under Article Tools).Citation: Polar Research 2015, 34, 24964, http://dx.doi.org/10.3402/polar.v34.24964

Sea-level changes in Iceland and the influence of the North Atlantic Oscillation during the last half millennium

We present a new, diatom-based sea-level reconstruction for Iceland spanning the last ∼500 years, and investigate the possible mechanisms driving the sea-level changes. A sea-level reconstruction from near the Icelandic low pressure system is important as it can improve understanding of ocean–atmosphere forcing on North Atlantic sea-level variability over multi-decadal to centennial timescales. Our reconstruction is from Viðarhólmi salt marsh in Snæfellsnes in western Iceland, a site from where we previously obtained a 2000-yr record based upon less precise sea-level indicators (salt-marsh foraminifera). The 20th century part of our record is corroborated by tide-gauge data from Reykjavik. Overall, the new reconstruction shows ca 0.6 m rise of relative sea level during the last four centuries, of which ca 0.2 m occurred during the 20th century. Low-amplitude and high-frequency sea-level variability is super-imposed on the pre-industrial long-term rising trend of 0.65 m per 1000 years. Most of the relative sea-level rise occurred in three distinct periods: AD 1620–1650, AD 1780–1850 and AD 1950–2000, with maximum rates of ∼3 ± 2 mm/yr during the latter two of these periods. Maximum rates were achieved at the end of large shifts (from negative to positive) of the winter North Atlantic Oscillation (NAO) Index as reconstructed from proxy data. Instrumental data demonstrate that a strong and sustained positive NAO (a deep Icelandic Low) generates setup on the west coast of Iceland resulting in rising sea levels. There is no strong evidence that the periods of rapid sea-level rise were caused by ocean mass changes, glacial isostatic adjustment or regional steric change. We suggest that wind forcing plays an important role in causing regional-scale coastal sea-level variability in the North Atlantic, not only on (multi-)annual timescales, but also on multi-decadal to centennial timescales.

Reconstructing the Last Glacial Maximum ice sheet in the Weddell Sea embayment, Antarctica, using numerical modelling constrained by field evidence

The Weddell Sea Embayment (WSE) sector of the Antarctic ice sheet has been suggested as a potential source for a period of rapid sea-level rise – Meltwater Pulse 1a, a 20 m rise in ∼500 years. Previous modelling attempts have predicted an extensive grounding line advance in the WSE, to the continental shelf break, leading to a large equivalent sea-level contribution for the sector. A range of recent field evidence suggests that the ice sheet elevation change in the WSE at the Last Glacial Maximum (LGM) is less than previously thought. This paper describes and discusses an ice flow modelling derived reconstruction of the LGM ice sheet in the WSE, constrained by the recent field evidence. The ice flow model reconstructions suggest that an ice sheet consistent with the field evidence does not support grounding line advance to the continental shelf break. A range of modelled ice sheet surfaces are instead produced, with different grounding line locations derived from a novel grounding line advance scheme. The ice sheet reconstructions which best fit the field constraints lead to a range of equivalent eustatic sea-level estimates between approximately 1.4 and 3 m for this sector. This paper describes the modelling procedure in detail, considers the assumptions and limitations associated with the modelling approach, and how the uncertainty may impact on the eustatic sea-level equivalent results for the WSE.

Punctuated eustatic sea-level rise in the early mid-Holocene

Whether eustatic sea-level rise through the Holocene has been punctuated or continuous has remained controversial for almost two decades. Resolving this debate has implications for predicting future responses of remaining ice sheets to climate change and also for understanding the drivers of human settlement and dispersal patterns through prehistory. Here we present a sea-level curve for the past 8900 yr from Singapore, a tectonically stable location remote from ice-loading effects. We also present critical and unique sedimentation rate, organic δ13C, and foraminiferal δ13C proxy records of sea-level change derived from a shallow-marine sediment core from the same area over the same time interval. The sea-level curve, corroborated by the independent proxy records, suggests rapid rise at a rate of 1.8 m/100 yr until 8100 cal (calibrated) yr B.P., a near cessation in the rate of sea-level rise between 7800 and 7400 cal yr B.P., followed by a renewed rise of 4–5 m that was complete by 6500 cal yr B.P. We suggest that this period of relatively stable sea level during the early to mid-Holocene enabled modern deltas to advance, providing a highly productive environment for the establishment of coastal sedentary agriculture. Periods of rapid sea-level rise before and after may have catalyzed significant postglacial episodes of human dispersal in coastal regions.

Geomorphology and benthic cover of mesophotic coral ecosystems of the upper insular slope of southwest Puerto Rico

The upper insular slope of southwest Puerto Rico is defined as extending from the shelf break at ~20 m water depth down to a depth of ~160 m where there is a pronounced change in geomorphic character and the basal slope begins. The upper slope is divided into two geomorphic zones separated by a pronounced break in slope gradient at ~90 m water depth. Descending from the shelf break, these are Zone I (20–90 m) and Zone II (90–160 m). As orientation of the shelf margin changes, geomorphology of Zone I shows systematic variations consistent with changes in exposure to prevailing waves. Within Zone I, exposed southeast-facing slopes have a gentler gradient and lower relief than more sheltered southwest-facing slopes, which are steep and irregular. Mesophotic coral ecosystems (MCEs) are largely restricted to Zone I and concentrated on topographic highs removed from the influence of active downslope sediment transport. Accordingly, MCEs are more abundant, extensive and diverse on southwest-facing slopes where irregular topography funnels downslope sediment transport into steep narrow grooves. MCEs are more sporadic and widely spaced on southeast-facing slopes where topographic highs are more widely spaced and downslope sediment transport is spread over open, low-relief slopes inhibiting coral recruitment and growth. Relict features formed during preexisting sea levels lower than present include deep buttresses at ~45–65 m water depth and a prominent terrace at ~80 m. Based on correlations with existing reef accretion and sea-level records, it is proposed that the 80-m terrace formed during the last deglaciation ~14–15 ka and subsequently drowned during a period of rapid sea-level rise associated with meltwater pulse 1A at ~14 ka and deep buttresses at ~45–65 m formed between ~11.5 and 13.5 ka and then drowned during a period of rapid sea-level rise associated with meltwater pulse 1B at ~11.3 ka.

Development of a mid-Holocene estuarine basin, Rhine–Meuse mouth area, offshore The Netherlands

A proper understanding of coastal development during periods of rapid sea-level rise is a prerequisite for the prediction of future coastal response to the expected acceleration in sea-level rise. However, the development of back-barrier basins, especially in river-mouth areas, during such periods is still not well understood. Here we show the response of back-barrier basins adjacent to the Rhine–Meuse river-mouth area, The Netherlands during the mid Holocene. A combination of high-resolution seismic data and cores was used to describe and explain late-Weichselian to early Holocene terrestrial and mid-Holocene back-barrier sequences. Along with dating and micropalaeontological analyses, these descriptions and explanations form the basis of a reconstruction of river-mouth and adjacent basin behaviour under conditions of rapid SLR. The nearby presence of the Rhine–Meuse estuary had a significant influence on the development of the adjacent back-barrier basin and especially its tidal channels. The back-barrier channels started to fill in between 8.3 and 7.4 ka BP due to decreasing rates of both sea-level rise and tidal-amplitude increase. During this time the Rhine relocated its mouth to the study area and the increased sediment delivery contributed to tidal-prism decrease and channel infill. Close to the Rhine–Meuse river mouth the back-barrier channels lay embedded in thick clayey, estuarine deposits and lateral migration of the tidal channels was limited. At greater distances, lateral migration rates increased as the thickness of cohesive back-barrier deposits decreases. The spatial pattern and stratigraphic setting of the northern channel fills suggests an episode of barrier overstepping between 7.5 and 6.6 ka BP. The mode and spatial limits of this overstepping are still not understood.

The sea-level record of the last deglacial in the Shinano River incised-valley fill, Echigo Plain, central Japan

The Echigo Plain, on the Japan Sea coast, is in a sedimentary basin governed by subsidence controlled by the Echigo Plain Western Margin Fault Zone. The glacial incised valley in the Echigo Plain filled with the sediment discharge of the Shinano River as the sea level rose during the last deglacial. The incised-valley fill allows us to reconstruct the detailed sea-level record, because the plain is still interseismically subsiding at a rate of about 3 mm/yr and the present tidal range along the Japan Sea coast is less than 0.3 m. In this study, we reconstructed a centennial-scale sea-level record for the deglacial period (12,000–9000 cal BP) on the basis of the sedimentary facies and 54 radiocarbon dates from a sediment core obtained from the Echigo Plain near the Western Margin Fault Zone. In this core, the interval from 95 to 55 m below the present sea level consists of floodplain and salt marsh sediments deposited aggradationally during 11,600–9000 cal BP as a result of transgression. Taking into account the Holocene subsidence rate of 2.7 mm/yr at the core site, the relative sea-level rise during the period of core deposition can be divided into the periods of rapid and slow rates. Sea level rose rapidly (> 23 mm/yr) during 11,600–11,100 cal BP, followed by a slow rate of sea-level rise (2 mm/yr) during 11,200–10,300 cal BP. Another period of rapid sea-level rise (11 mm/yr) during 10,500–9400 cal BP was followed by a slow rate of rise (2 mm/yr) during 9600–9000 cal BP. This record of relative sea-level rise indicates that the rapid rate of sea-level rise during 11,600–11,100 cal BP is consistent with meltwater pulse 1B.

Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma)

Carbon stable-isotope variation through the Cenomanian–Santonian stages is characterized using data for 1769 bulk pelagic carbonate samples collected from seven Chalk successions in England. The sections show consistent stratigraphic trends and δ13C values that provide a basis for high-resolution correlation. Positive and negative δ13C excursions and inflection points on the isotope profiles are used to define 72 isotope events. Key markers are provided by positive δ13C excursions of up to +2‰: the Albian/Cenomanian Boundary Event; Mid-Cenomanian Event I; the Cenomanian/Turonian Boundary Event; the Bridgewick, Hitch Wood and Navigation events of Late Turonian age; and the Santonian/Campanian Boundary Event. Isotope events are isochronous within a framework provided by macrofossil datum levels and bentonite horizons. An age-calibrated composite δ13C reference curve and an isotope event stratigraphy are constructed using data from the English Chalk. The isotope stratigraphy is applied to successions in Germany, France, Spain and Italy. Correlation with pelagic sections at Gubbio, central Italy, demonstrates general agreement between biostratigraphic and chemostratigraphic criteria in the Cenomanian–Turonian stages, confirming established relationships between Tethyan planktonic foraminiferal and Boreal macrofossil biozonations. Correlation of the Coniacian–Santonian stages is less clear cut: magnetostratigraphic evidence for placing the base of Chron 33r near the base of the Upper Santonian is in good agreement with the carbon-iso-tope correlation, but generates significant anomalies regarding the placement of the Santonian and Campanian stage boundaries with respect to Tethyan planktonic foraminiferal and nannofossil zones. Isotope stratigraphy offers a more reliable criterion for detailed correlation of Cenomanian–Santonian strata than biostratigraphy. With the addition of Campanian δ13C data from one of the English sections, a composite Cenomanian–Campanian age-calibrated reference curve is presented that can be utilized in future chemostratigraphic studies.The Cenomanian–Campanian carbon-isotope curve is remarkably similar in shape to supposedly eustatic sea-level curves: increasing δ13C values accompanying sea-level rise associated with transgression, and falling δ13C values characterizing sea-level fall and regression. The correlation between carbon isotopes and sea-level is explained by variations in epicontinental sea area affecting organic-matter burial fluxes: increasing shallow sea-floor area and increased accommodation space accompanying sea-level rise allowed more efficient burial of marine organic matter, with the preferential removal of 12C from the marine carbon reservoir. During sea-level fall, reduced seafloor area, marine erosion of previously deposited sediments, and exposure of basin margins led to reduced organic-carbon burial fluxes and oxidation of previously deposited organic matter, causing falling δ13C values. Additionally, drowning of carbonate platforms during periods of rapid sea-level rise may have reduced the global inorganic relative to the organic carbon flux, further enhancing δ13C values, while renewed platform growth during late transgressions and highstands prompted increased carbonate deposition. Variations in nutrient supply, changing rates of oceanic turnover, and the sequestration or liberation of methane from gas hydrates may also have played a role in controlling carbon-isotope ratios.

Late Cretaceous (Campanian) carbon isotope events, sea-level change and correlation of the Tethyan and Boreal realms

Carbon stable-isotope stratigraphy provides unique insights into environmental change during the Campanian, the longest (83.5–71.3 Ma), but least well-understood stage of the Late Cretaceous. A new carbon isotope (δ 13C) profile for a 500-m-thick Campanian–basal Maastrichtian Tethyan pelagic–hemipelagic section near El Kef, northern Tunisia, is calibrated using data from a biostratigraphically well-constrained succession at Kalaat Senan. The general shapes of the Tunisian δ 13C reference curve and published Tethyan δ 13C profiles from Elles (Tunisia) and Bidart (SW France), and a Boreal curve for the Trunch borehole (eastern England), are remarkably similar in all three areas. A positive carbon isotope event of +0.2‰ δ 13C in the mid-Campanian dated at 78.7 Ma and a negative excursion of −0.4‰ in the upper Campanian at 74.8 Ma can be correlated between Tunisia and England. A positive excursion of +0.3‰ at 83.7 Ma spans the Santonian–Campanian boundary. These isotope events enable precise inter-regional correlations that are consistent with published nannofossil data. Review of Campanian sea-level data from North Africa, the Middle East and northern Europe indicates that major shifts in δ 13C profiles coincide with changes in eustatic sea-level. Relatively stable δ 13C values in the lower Campanian and their long-term fall through the upper Campanian reflect high and then falling eustatic sea-levels, and increased carbonate production. Short-term (∼600 kyr) positive excursions record greater organic productivity and/or organic matter preservation, and decreased carbonate fluxes during periods of rapid sea-level rise and the drowning of carbonate platforms. Excursions were terminated by falling nutrient supply and increased carbonate deposition associated with epicontinental sea expansion and renewed carbonate platform growth during the late transgression and highstand. Negative excursions are linked principally to reworking of marine and terrestrial organic matter during rapid sea-level fall. Carbon isotope stratigraphy is a powerful tool for correlation which can be used to test the validity of Campanian global biostratigraphic frameworks, and improve our understanding of the nature and timing of Late Cretaceous sea-level change.

太平洋島しょ域におけるマングローブ林の立地形成と海水準変動

Geomorphological evolutions of mangrove habitats on the Pacific islands and prediction of the habitat dynamics were discussed using the examples of Kosrae Island in Micronesia, Bohole Island in the Philippines and Iriomote and Ishigaki Islands belonging to the Yaeyama Islands in Japan. Most of the mangrove forests of Kosrae have been developed during the last 2000 years by accumulating mangrove peat with the gradual sea-level rise of 1 to 2 mm/yr. During the period of rapid sea-level rise of about 10 mm/yr between 4100 and 3700 yr B.P., the mangrove forests ceased peat accumulation and retreated landward. These evidences suggest that the critical rate of mangrove peat accretion with sea-level rise is between 2 mm/yr and 10 mm/yr. If the velocity of the anticipated sealevel rise exceeds the critical rate, all of the mangrove forests of Kosrae will retreat landward and reduce rapidly. The mangrove habitats on Bohole Island have been formed by the same processes as on Kosrae Island except the expansion of clay layer overlying a mangrove peat layer during the last several hundred years. The clay deposit, which was inferred to have derived from the deforested slopes, supported Nypa fruticans thicket. Rapid sea-level rise over the critical rate of the mangrove peat accumulation will induce the retreat of mangrove forests from their seaward margin, however, the invasion of mangrove trees into the Nypa habitat will not occur immediately because the habitat conditions of the clay deposited area is no longer suitable for the growth of mangrove species except Nypa. Mangrove habitats of Iriomote and Ishigaki Islands abruptly moved seaward by a relative sea-level fall that occurred about 1000 and 2000 yr B.P., apparently caused by seismic uplift. The thickness of the layers containing mangrove organic materials in present mangrove habitats were less than 1 m. The organic layers have deposited during the last several hundred years. During the last few decades, input of sediments caused by soil erosion has affected the seaward expansion of mangrove forests. However, the sediments supply will result the reduction of mangrove area because the seaward limit of mangrove expansion is restricted by the geomorphological framework.

Mangrove Habitat Formation and Response to Holocene Sea-level Changes on Kosrae Island, Micronesia

Mangrove habitats on Kosrae are divided into three types, i.e., an estuary or delta type, a backmarsh or lagoon type and a coral reef or tidal-flat type. Most of the mangrove forests of Kosrae have been developed during the last 2000 years by accumulating mangrove peat with the gradual sea-level rise of 1 to 2 mm/yr except the landward part of the estuary or delta type. On the other hand, during the period of rapid sea-level rise of about 10 mm/yr between 4100 and 3700 yr B.P., the mangrove forests ceased peat accumulation and retreated landward. Until 3500 yr B.P., mangrove forests were distributed only in narrow bands in the inlets. Therefore, the critical rate of mangrove peat accretion with sea-level rise is estimated at more than 2 mm/yr and less than 10 mm/yr. If the anticipated sea-level rise exceeds this critical rate, all of the mangrove forests of Kosrae will retreat landward and reduce rapidly.