Late Holocene Sea-level Fall Research Articles

Holocene barrier overstepping, estuarine rollover and drainage merging in a sub-tropical bay

A combination of abundant accommodation and high rates of sediment delivery can allow estuarine incised valleys to archive post-glacial coastal evolutionary processes, including the effects of geological controls on valley positioning and stratigraphic evolution. Seismic-stratigraphic and vibracoring investigations in Babitonga Bay, Santa Catarina, Brazil revealed four main stratigraphic units related to the Upper Pleistocene and Holocene Transgressive System Tract (TST) and Highstand System Tract (HST). The interior of Babitonga Bay was subaerially exposed during lower sea levels, forming two distinct incised valleys. The first was established over the Palmital Shear Zone, while the second was formed beneath the modern Main Channel. During the period of Holocene sea-level rise, a coastal lagoon-barrier system was formed and overstepped, leaving remnants on the shallow shelf. Estuarine facies migrated landward until the drainage systems of the adjacent valleys were unified into the single estuarine basin of Babitonga Bay. During late Holocene sea-level fall, the back-barrier lagoon was infilled and prograding barriers and sandy spits developed in estuarine mouths. This study captures the unique geological framework and post-glacial evolutionary history of Babitonga Bay, illustrating the evolution of a stable tectonic valley in response to rising sea level and changing coastal morphodynamics during the subsequent phase of sea-level fall. It broadens our understanding of the response of similar coupled estuary / coastal sedimentary systems environments in response to sea-level change.

Mid-Late Holocene Paleoenvironmental and Sea Level Reconstruction on the Al Lith Red Sea Coast, Saudi Arabia

Late Quaternary paleoenvironments are of particular interest to understand how the Earth System’s climate will respond to the undramatic changes during this period, compared with the broader glacial-interglacial variations. In this study, a shallow sediment core (2.84 m long) retrieved from the Red Sea coastal zone in northern Ghubbat al Mahasin, south of Al-Lith, Saudi Arabia, is used to reconstruct the mid-Late Holocene paleoenvironments and sea level based on a multiproxy approach. Remote sensing data, sedimentary facies, benthic foraminiferal assemblages, δ18O and δ13C stable isotopes, elemental composition and14C dating were utilized. The stratigraphy of the core shows three distinctive depositional units. The basal pre 6000 year BP unit consists of unfossiliferous fine to medium sand sharply overlain by black carbonaceous mud and peat, suggesting deposition in a coastal/flood plain under a warm and humid climate. The middle unit (6000-3700 year BP) records the start and end of the marine transgression in this area. It consists of gray argillaceous sand containing bivalve and gastropod shell fragments and a benthic foraminiferal assemblage attesting a lagoonal or quiet shallow marine environment. The upper unit (<3700 year BP) consists of unfossiliferous yellowish-brown argillaceous fine-grained sands deposited on an intertidal flat. Both middle-and upper-units stack in a regressive shallowing upward pattern although they may be separated by a hiatus. The overall regressive facies and the stable isotopic data are consistent with a late Holocene sea-level fall and a change to a more arid climate.

Past environmental changes during the Late Holocene sea-level fall (last 2.7 Ka) at Bahía Samborombón, NE coastal plain (Argentina)

Here we present a reconstruction of vegetation history and environmental changes at the central zone of Bahía Samborombón during the Late Holocene sea-level regressive phase (last 2.7 Ka) corresponding to MIS 1. This study is an integrative approach based on the combination of geomorphological and micropaleontological (pollen, non-pollen palynomorphs and foraminifera) proxy data. We found agreement among the environmental inferences from each proxy, which clearly show the evolution of the estuarine saltmarsh from an intertidal environment under significant marine influence towards a supratidal brackish-freshwater environment as sea-level was falling. The evidence of this evolution is (1) coastal progradation encompassing at least 5.7 km as revealed by relict shorelines. Saltmarsh elevation is another evidenced as a result of accretion due to fine sediments accumulation of at least 1.2 m thick. We posit based on these values, significant progradation and accumulation processes. (2) Plant succession, characterized by the replacement of low saltmarsh vegetation by that typical of high saltmarsh; which then is associated to brackish-freshwaters plant communities in shallow water-bodies environments. (3) A gradual decrease of dinocysts and foraminifera assemblages, the latter absent during the last millennium indicating a decrease in marine influence and the consequent lower salinity values. Bahía Samborombón coastal plains became a regional scale character during the last 2.7 Ka due to its surface increase, turning into one of the most extensive southeastern-South America saltmarshes, currently constituting a biosphere reserve.

Timing, magnitude and effects of late Holocene sea level drawdown on island habitability, Aitutaki, Cook Islands

ABSTRACTGeologist Bill Dickinson argued that prior to late Holocene sea level fall, in many Pacific island settings low‐lying islands were awash, shallow nearshore environments were restricted and human settlement was constrained or sometimes impossible. Stable coastlines and islets of modern configuration only developed after the “cross‐over date”, when declining high‐tide levels fell below mid‐Holocene low‐tide levels, a regionally variable process. We evaluate evidence from the almost‐atoll of Aitutaki, Cook Islands against this model, providing: (1) a local late Holocene sea level reconstruction including nine U/Th‐dated microatolls; (2) 22 new AMS dates on human activities, many from small, low‐lying offshore islets; and (3) elevation data for 14C‐dated cultural deposits on three islets. Our results include an early first millennium sea level position 0.74–0.97 m (± 0.126) above modern height‐of‐living‐corals, an eighth to eleventh century AD minimum relative to the long‐term trend, and a sea level rise peaking in the mid‐fourteenth to sixteenth centuries. This reconstruction, combined with twelfth century AD Cocos nucifera charcoal, informs on the timing and distribution of human activities across Aitutaki's evolving land and seascapes and sea level impacts. While our findings do not contradict Dickinson's model of sea level constrained island settlement, other explanations cannot be excluded.

Holocene evolution of the Xagó dune field (Asturias, NW Spain) reconstructed by means of morphological mapping and ground penetrating radar surveys

Morphological mapping and ground penetrating radar (GPR) profiling were carried out in the Xago aeolian dune field along the Asturias coast of NW Spain to reconstruct its Holocene evolution. Such data provide a much more accurate picture than can be inferred from surficial morphological studies alone. Three successive dune sequences were identified: an inner (climbing dunes), a middle (large transverse ridge and minor elongated dunes) and an outer dune field (foredune with lee-projection dunes and incipient foredune). A late Holocene sea-level fall is inferred from the relative position of the dunes together with a prograding tendency. Long intervals of stabilisation, during which each dune sequence was formed, are interspersed within the deposit. The GPR records also reveal a period of erosion in the southern middle field, which was followed by accretion. The results show that both progradational and erosional processes occurred during the Holocene evolution of the dune field, features that can be extended to other dune fields in similar settings at these latitudes. Stratigraphically, the Xago dune field is an excellent example where internal reflectors reveal an erosion surface representing a transgressive or sea-level stillstand event that had previously remained undetected.

Chronostratigraphy of Bramston Reef reveals a long-term record of fringing reef growth under muddy conditions in the central Great Barrier Reef

Inshore reefs on Australia's Great Barrier Reef (GBR) are widely argued to be in decline, although recent reports suggest that some may be more resilient than traditionally assumed. Resolution of this debate requires long-term insights into past reef development and variability to provide context for the assessment of present reef condition. Long-term reef growth histories can preserve extended records of reef growth and condition, however they are rare, especially for mainland-attached fringing reefs, which are themselves uncommon on the GBR. We examined the internal structure and ecology at Bramston Reef, a mainland-attached fringing reef located in a protected bay on the central GBR. Eight reef matrix cores were collected across the reef flat. Sedimentological and palaeo-ecological analyses coupled with U–Th dating were used to develop the first reef growth history for a shore-attached fringing reef in this region. Twenty-five hard coral genera were identified in the palaeo-ecological analyses. The key reef-building genera (including Acropora, Montipora, Euphyllia, Porites and Goniopora) have contributed to reef growth since initiation and are represented in the extant coral community, despite a change in accretion ‘mode’ during the late Holocene. Sedimentological and stratigraphic investigations demonstrate Bramston Reef has always grown in a mud-rich setting. U–Th ages indicate that reef initiation occurred at or before 5400yBP in a palaeo-water depth of 2–3m. Bramston Reef reached sea level by 4256yBP when sea level was approximately 1m higher than present, after which rapid seaward progradation occurred until around 3000yBP (~19cm/yr on average). Between approximately 3000 and 1000yBP seaward progradation of the reef flat slowed (to ~9.8cm/yr on average). This deceleration of reef growth occurred long before European settlement of the Queensland coast and was driven by natural constraints, probably associated with limited accommodation space due to late-Holocene sea-level fall. Our results demonstrate that mainland-attached reef initiation and accretion can occur in muddy inshore environments over long timeframes (centuries to millennia).

Late Holocene sea-level fall and turn-off of reef flat carbonate production: Rethinking bucket fill and coral reef growth models

Relative sea-level rise has been a major factor driving the evolution of reef systems during the Holocene. Most models of reef evolution suggest that reefs preferentially grow vertically during rising sea level then laterally from windward to leeward, once the reef flat reaches sea level. Continuous lagoonal sedimentation (bucket fill) and sand apron progradation eventually lead to reef systems with totally filled lagoons. Lagoonal infilling of One Tree Reef (southern Great Barrier Reef) through sand apron accretion was examined in the context of late Holocene relative sea-level change. This analysis was conducted using sedimentological and digital terrain data supported by 50 radiocarbon ages from fossil microatolls, buried patch reefs, foraminifera and shells in sediment cores, and recalibrated previously published radiocarbon ages. This data set challenges the conceptual model of geologically continuous sediment infill during the Holocene through sand apron accretion. Rapid sand apron accretion occurred between 6000 and 3000 calibrated yr before present B.P. (cal. yr B.P.); followed by only small amounts of sedimentation between 3000 cal. yr B.P. and present, with no significant sand apron accretion in the past 2 k.y. This hiatus in sediment infill coincides with a sea-level fall of ~1-1.3 m during the late Holocene (ca. 2000 cal. yr B.P.), which would have caused the turn-off of highly productive live coral growth on the reef flats currently dominated by less productive rubble and algal flats, resulting in a reduced sediment input to backreef environments and the cessation in sand apron accretion. Given that relative sea-level variations of ~1 m were common throughout the Holocene, we suggest that this mode of sand apron development and carbonate production is applicable to most reef systems.

Mid and late Holocene multiproxy analysis of environmental changes linked to sea-level fluctuation and climate variability of the Río de la Plata estuary

In the present study, we carried out pollen, diatom and other palynomorph analyses from a sedimentary sequence from the marshes of Arroyo Solís Grande, in the Río de la Plata northeastern coast (Uruguay), and compared the results with a regional model of surface pollen–vegetation relationship from the Atlantic coastal salt marshes (30°–37° S). Results indicated that salt marsh vegetation developed around the estuary between 8000 and 5100calyr BP, as the sea level rose and reached the Holocene sea-level highstand. Brackish marshes around the estuary between 5100 and 2900calyr BP reflect the late Holocene sea-level fall and associated increased input of freshwater. Brackish marshes and shallow salt pond environments between 2900 and 1000calyr BP are most probably linked to the Paraná delta formation and a further sea-level fall. Present-day marshes behind the sand bar characterize the last 1000calyr BP. The integration of our results with multiproxy records from the Río de la Plata southern coast (Argentina) and from the coastal plain of the southern Atlantic margin of South America (30°–37° S) revealed that marine influence during the maximum sea-level highstand resulted in the development of vegetation similar to that of the geographical region. However, after ca. 3000calyr BP, local differentiation related to different climate and geomorphology occurred at each area, thus resulting in the establishment of different modern plant communities at different times during the late Holocene.

A multi-proxy study of Holocene environmental change recorded in alluvial deposits along the southern coast of the Pampa region, Argentina

Analyses of fossil records of molluscs, stable isotopes of carbon and oxygen, diatoms and palynomorphs from alluvial deposits at Arroyo Claromeco � , in the southern Pampa region of Argentina, provide an integrated reconstruction of rapid Holocene environmental change related to marine and palaeoclimate influences. The major events recorded include: (i) disturbances produced by marine influences in close proximity to the coast before ca. 7700 cal a BP; (ii) a short-term decrease in salinity associated with freshwater influx between ca. 7700 and 7400 cal a BP in response to eustatic adjustment as sea level rose; (iii) an increasing input of marine water related to the Holocene sea-level highstand and greater evaporative conditions between 7400 and 6450 cal a BP; and (iv) evaporative processes linked to a pronounced climatic variability that triggered changes in water chemistry during the last ca. 6000 cal a BP concurrently with the late Holocene sea-level fall. A comparison of coastal and continental stable isotope records from the Pampa region reveals that evaporation and a negative precipitation/evaporation (P/E) ratio balance strongly influenced water chemistry in the coastal region, and was as important an influence as marine water during the period of maximum sea-level highstand. Copyright # 2014 John Wiley & Sons, Ltd.

Magnetic fingerprint of the late Holocene inception of the Río de la Plata plume onto the southeast Brazilian shelf

Measurement of changes in the assembly of magnetic particles in sediment records is useful both for paleoclimatic and paleoceanographic reconstructions and for assessing provenance and pathways of sediment transport. In this study we provide detailed rock magnetic analysis along with scanning electron microscopy (SEM), Ti/Ca ratio and particle size distribution of the decarbonated fraction of sediments to trace terrigenous source changes during the last 6calkyrBP in core 7610 collected on the SE Brazilian continental shelf. Magnetic mineralogy data indicate a gradual shift between two distinct sources of sediments: (1) dust from southern South America (Patagonia, central western Argentina and Puna-Altiplano plateau) and (2) sediments eroded from the weathered volcanics of Paraná Basin. These materials reached the continental margin off Argentina and Uruguay by different pathways and were transported northward by the Brazilian Coastal Current (BCC). The chronology of events related to the environmental record of core 7610 indicates that between 6 and 4.7calkyrBP dry conditions over continental South America resulted in enhanced eastward eolian transport of the Argentinean loess and a limited discharge of the RdlP river load into the Atlantic Ocean. At this time, the sedimentary magnetic fraction that reached the SE Brazilian shelf reflect predominantly loessic sources transported from the Pampean terraces to the Argentinean and Uruguay shelves and then distributed northward by the shelf current system. From 4.7 to 2calkyrBP the climate became gradually wetter over the continent and the contribution of oxidized phases from the weathered Paraná Basin (mainly from the basaltic and acidic volcanics) started to become important. This change in climate overlaps in time a with the late Holocene sea level fall recorded along the SE South American coast. Later, from 2 up to 0.9calkyrBP even wetter conditions were prevalent in S and SE South America, and the RdlP sedimentary load became the main source of sediments transported to the SE Brazilian shelf by the BCC. The magnetic record of core 7610 reported here together with other paleoenvironmental tracers (palynology, geochemistry and stable isotopes) thus indicates that the inception of the RdlP plume onto the SE Brazilian shelf started as early as 4.7calkyrBP and has been strengthened since then mostly due to changes in precipitation over South America and to a smaller extent by sea-level changes.

Evolution of a coquina barrier in Shark Bay, Australia by GPR imaging: Architecture of a Holocene reservoir analog

The Holocene hypersaline carbonate system in Shark Bay, Australia is represented by microbial deposits (stromatolites and thrombolites) and a supratidal coquina beach ridge system which overlies the Pleistocene Bibra Formation and is prograding seaward, since 4500years ago, over supratidal–intertidal microbial deposits as a consequence of Late Holocene sea level fall, and the high volume of bivalve shells available. The extent, internal architecture and ridge constructional types of coquina deposits in this World Heritage area have not been previously investigated in detail by cores tied with geophysical images. Here we document external and internal architecture of the Holocene coquina system in southeast Hamelin Pool showing three depositional units which comprise the ridge system construction: (1) tabular layers complex, (2) convex-up ridges and (3) washover deposits. Bivalve skeletons represent almost the total amount of bioclasts with symmetrical semi-circular formats and valve sizes 5–9mm. Based on Ground Penetrating Radar (GPR) images the number of prograding layers present in each transect line varies from 252 to 433, which when compared with the measured 14C ages, shows lateral growth of the system at average rates from 10years per layer on proximal older sequences to 30years per layer in the younger and thicker packages which display more gentle inclination and sub-horizontal arrangement. Divergent orientation of tabular layers relates to changes in energy of currents and waves and involves time gaps and erosion. After earliest crest construction the system experienced a decrease in sea level (3600–3000 14C years BP) that left behind a sequence of parallel tabular layers with dip angles (>10° to <20°) which accreted seaward as the first major prograding event. The accretionary tabular layers progressively lost height till near 3000years BP, representing continuous relative SL and storm energy fall, when the first major swale was constructed. Between 2000 and 3000years BP a series of events constructed 4 large convex-up crests separated by swales overlying tabular layers. Marked changes in layer arrangement occurred since 1000years ago when subsequent to erosional events the coastal system rearranged and is characterized by layers with low dip angle (<10°). The Shark Bay ridge system represents the results of waves, surges and swash during storm activity and subsequent reworking history of eroded material in a context of continuous sea level fall. Alternating beach ridges and swales represent events of storm intensity variation during falling late Holocene SL. This progradational model of coquina ridge construction at Shark Bay is a useful analog for ancient coquina reservoirs deposited in lacustrine systems, since the hydrodynamic setting and semi-closed environment affected by storms is similar to some ancient examples such as the coquina reservoirs in the Campos Basin, Brazil.

Island ecosystem and biodiversity dynamics in northeastern Australia during the Holocene: Unravelling short-term impacts and long-term drivers

Vegetation changes of tropical Lizard Island (Great Barrier Reef World Heritage Area, Australia) over the last 8000 years are derived from palaeoenvironmental analysis of a 475 cm long sediment core. During early-Holocene sea-level rise, flooding of the continental shelf and thus isolation of Lizard Island, the pollen record shows the gradual establishment of a mangrove forest paralleled by contraction of the near-coastal palm and grass-dominated vegetation. Subsequently, mid-Holocene relative environmental stability supported a diverse, Rhizophora-dominated mangrove and open, mixed sclerophyll vegetation inland. Around 6000 years ago, a profound disturbance of the mangrove is recorded by a siliciclastic layer and we hypothesise that this deposit documents the impact of a storm or cyclone. Postevent environmental conditions were strongly altered with enhanced estuarine conditions supporting a Sonneratia and Bruguiera-dominated mangrove forest. During late-Holocene sea-level fall and stabilisation, progradation and contraction of the mangrove forest was paralleled by the expansion of a palm-dominated swamp. Freshwater taxa continued to dominate the record, however, a distinct disturbance signal from anthropogenic activity is recorded in the last century. Although Sonneratia dominated the post-event mangrove, late-Holocene environmental instability led to the extinction of this genus on the island. Local environmental changes in the freshwater swamp and rainforest also led to the loss of Arenga and Ilex from the island’s ecosystems. Our record implies that long-term ecosystem and biodiversity change on Lizard Island is: (a) primarily reflected in the spatial extent of the island’s vegetation communities and the species dominance within them and (b) driven by an interplay between climate, sea-level and potentially human activity. In addition, a short-term impact provoked the reconfiguration of the mangrove, potentially causing long-term ecosystem instability and thus impacting on mangrove biodiversity development on the Great Barrier Reef islands.

Development of an inshore fringing coral reef using textural, compositional and stratigraphic data from Magnetic Island, Great Barrier Reef, Australia

The stratigraphy of fringing coral reef environments and platforms provides unique insights into reef development and evolution. This study used twelve sediment cores from three transects across a reef flat in Nelly Bay, Magnetic Island (NE Australia), to examine its development over the Holocene. The maximum thickness of the Holocene reefal material based on seismic and bore log data was around 5m and comparable to other fringing reefs in the Great Barrier Reef (GBR). Six different sedimentary facies were identified in the cores and two coincided with reef accretion: the lower muddy sand with coral rubble and the upper siliciclastic sand with coral rubble. Radiocarbon and U-series dates show that the Nelly Bay fringing reef initiated around 6300yr BP on a gently sloping, unconsolidated or weakly cemented Pleistocene alluvial sedimentary facies. The ages of four massive coral heads (range between 5790 and 6290 calibrated C-14 yr BP) were almost contemporaneous with reefal initiation indicating that conditions were favourable for reef growth around this time. Vertical accretion rates for the Nelly Bay fringing reef were, on average about 0.5mm/yr over the last 6000yr, although rates were as high as 5mm/yr during initiation. Average lateral accretion rates varied from 98 to 120mm/yr, which are comparable to rates of other fringing reefs in the region. The age structure of the lower muddy sedimentary facies was consistent with the classic seaward prograding model of fringing reef development. In contrast, the upper siliciclastic sand with coral rubble facies was much younger in age (<1100yr BP) than the models and appears to represent an erosional unconformity between the upper and lower reefal facies possibly linked to late Holocene sea-level fall. This younger facies extends laterally across the reef flat and has no obvious prograding symmetry. Our integrated approach, using multi-proxy analyses of sediment cores from the Nelly Bay reef flat, highlights the complex development of this reef which include changes in coral composition and rubble preservation, reef evolution in response to sea-level change and changes in reef accretion rates.

Significance of subtidal microbial deposits in Shark Bay, Australia

Microbial deposits at Shark Bay, Australia constitute one of the largest and most diverse modern occurrences around the world. The microbial carbonate system has developed in response to environmental change from near open marine into restricted embayment with hypersalinity and high evaporation. Bacterial sediments started depositing at about 2000 years BP, after the Holocene maximum flooding of sea level about 6800 years BP, due to increasing salinity as the sea level fell by about 2.5 m, within the Holocene highstand system tract. The extent and types of subtidal microbial substrate in this World Heritage area have not previously been fully systematically mapped and described. Here we document the importance of subtidal microbial deposits occupying as much as 10 times the occurrence area of the intertidal microbial mats and stromatolites and demonstrate that deeper structures have a cerebroid external morphology with a non laminated internal fabric. Subtidal structures contain consistently different internal fabrics with carbonate grains and fragments interbedded with aragonitic micrite arranged in millimetric laminae or sub-spherical patches, which display fabrics such as well laminated (Smooth), coarse laminated wavy (Colloform), sub-laminar to irregular with voids and biofragments of bivalves (Microbial Pavement), irregular clotted (Pustular) and non-laminated (Cerebroid), constituting carbonate sediment with stromatolitic facies (Smooth and Colloform), thrombolitic facies (Pustular) and cryptomicrobial non-laminated (Cerebroid and Microbial Pavement). Each microbial structure grows in specific settings according to water depth (MSL), with Cerebroid build-ups commonly occurring between − 1 and − 2.5 m and Microbial Pavement extending as deep as 6 m. Because of the late Holocene sea level fall microbial structures often record in their internal features a shallowing-up carbonate cycle. These newly documented extensive subtidal microbial structures, with distinctive morphologies and fabrics have importance as modern occurrences and provide potential analogues for understanding ancient microbial systems and environments.

Demise of reef-flat carbonate accumulation with late Holocene sea-level fall: evidence from Molokai, Hawaii

Twelve cores from the protected reef-flat of Molokai revealed that carbonate sediment accumulation, ranging from 3 mm year−1 to less than 1 mm year−1, ended on average 2,500 years ago. Modern sediment is present as a mobile surface veneer but is not trapped within the reef framework. This finding is consistent with the arrest of deposition at the end of the mid-Holocene highstand, known locally as the “Kapapa Stand of the Sea,” ~2 m above the present datum ca. 3,500 years ago in the main Hawaiian Islands. Subsequent erosion, non-deposition, and/or a lack of rigid binding were probable factors leading to the lack of reef-flat accumulation during the late Holocene sea-level fall. Given anticipated climate changes, increased sedimentation of reef-flat environments is to be expected as a consequence of higher sea level.

Mid-Holocene evolution and paleoenvironments of the shoreface–offshore transition, north-eastern Argentina: New evidence based on benthic microfauna

A sedimentary record spanning 5792–5511 cal yr BP and 3188–2854 cal yr BP was recovered at 36° 45′ 43″ S–56 ° 37′ 13″ W, south-west South Atlantic. The sedimentological features and micropaleontological (benthic foraminifera and ostracoda) content were analyzed in order to reconstruct paleoenvironmental conditions. Considerable environmental fluctuations are indicated by all these proxies. Five different stages were distinguished: Stage 1 (ca. 5800–5000 cal yr BP) consists of muddy sand with abundant microfossils. In this interval, species typical for inner marine shelf environments maintained a high abundance. Stage 2 consists of plastic light greenish grey clays barren of microfossils, and probably represents fluvial input from the de la Plata River to the shelf contemporaneous of a lowering of sea level. Stage 3 is composed of brownish yellow sandy silts, and represents increasing marine conditions in the area as reflected by higher faunal diversity and typical foraminifera of inner shelf environments. Stage 4 is made of homogeneous mud, barren of microfossil, which represents a new pulse of fluvial input to the shelf in consequence of a new fall in sea level. The final part of the core (Stage 5) is a coarsening upward sequence, grading from greeny brown clayey sandy silts to coarse shelly sands and represents the modern sedimentation in the area. This interpretation strengthens the stepped model of late-Holocene sea-level fall between 5511–5792 cal yr BP and 2854–3188 cal yr BP in Buenos Aires coast, and agrees with the relative sea-level history previously proposed by some authors from western South Atlantic coasts.

A critical review of mid- to late-Holocene sea-level fluctuations on the eastern Brazilian coastline

The eastern coast of Brazil is 7000 km long and has been the subject of systematic paleo-sea-level investigations for more than 35 years. More than 1000 samples have been radiocarbon dated, and paleo-sea-level trends have been determined for 14 coastal sectors. These trends have in common a mid-Holocene sea-level maximum (PMT) above present sea-level and a subsequent fall to the present time. The time and elevation of the PMT, the time when relative sea-level rose above present mean sea-level, and the nature of late Holocene sea-level fall can, however, differ significantly. Discrepancies are observed not only between neighboring coastal sectors, but also between different studies in a same coastal sector. This paper discusses all the key radiocarbon-dated samples used to establish regional trends in relative sea-level on the eastern coast of Brazil. It is concluded that although many of the key sea-level indicators are imprecise (in time and space), there is widespread evidence (30% of the data set) for a progressive decline, possibly with uneven rates, of relative sea-level since the end of the PMT.

Holocene sea-level changes in the Indo-Pacific

Holocene sea-level reconstructions exist from many locations in the Indo-Pacific region. Despite being a large geographical region, the nature of Holocene sea-level change is broadly similar in all locations. Differences do exist, however, in the timing and magnitude of the Mid-Holocene High Stand (MHHS) and the nature of late Holocene sea level fall across the region. When the Indo-Pacific is subdivided into smaller regions, these discrepancies do not disappear, and in some cases the discrepancies are large within a single coastline. It is clear from this analysis that the fundamental criteria to produce accurate local relative sea-level curves are hardly ever met. There are serious problems associated with the correct interpretation of sea-level indicators and their relationship to mean sea level, and with the quality of age determinations. A consistent methodology throughout the Indo-Pacific for the analysis of sea level data is lacking. Future sea-level analysis from far field locations must involve the application of a consistent methodology in order to allow meaningful comparison between studies. This should help to resolve the ongoing debate about the magnitude and timing of the Mid-Holocene High Stand, and the nature of late Holocene sea-level fall across the region.

Environmental and sea-level variations on the southeastern Brazilian coast during the Late Holocene with comments on prehistoric human occupation

Analysis of organic sediments, including their diatom and pollen contents, was carried out in a swamp forest on the coastal plain of São Paulo State (southeastern Brazil). A palaeoenvironmental reconstruction was made regarding this region where communities of fisher–gatherers lived in prehistory, producing numerous shell mounds. From at least 4900 until 3470 cal yr BP the site was a lagoon surrounded by a relatively open forest. The occurrence of shell mounds 50 km from the coast and the ‘peak’ of human occupation between 5700 and 3300 cal yr BP can be linked to the presence of this great lagoon which formed an important food source. The lagoon desiccated ca. 3470 cal yr BP, i.e. during the Late Holocene sea-level fall, and was replaced by a swamp forest that still exists today. This forest was altered during a short period between 1300 and 675 cal yr BP, when a freshwater lake or marsh was formed as a result of a rise in groundwater level. The climate in the area throughout the Middle–Late Holocene (ca. 5000 cal yr BP to the present) has not changed significantly except for three slightly more humid episodes at 3470–2750, ca. 1940, and 1300–675 cal yr BP. No indications of human impact on the vegetation during this period could be identified. Our results show that from 4900 to 3470 cal yr BP sea-level was higher than at present, contradicting a widely accepted relative sea-level trend that suggests that at least two ‘negative’ sea-level oscillations occurred during the Holocene, i.e. at 4560–4150 and 3200–2800 cal yr BP. Our data confirm recent studies instead proposing a continuously declining relative sea-level during the Late Holocene.

Effect of Late Holocene sea-level fall on atoll development

Abstract A net fall of 2.4 m since 2760 yrs B.P., the local date for the Flandrian Transgression Maximum, has been of major importance in reef and atoll islet growth in the Gilbert and Ellice Islands. The islets have been built from sand taken from the shallow margins of the lagoonal floor as a direct result of this fall, and it may be no coincidence that the earliest Micronesian and Polynesian settlements date from about the sea level maximum. Later second-order transgressions have almost certainly been important in the development of zonation and increased width of the low-tide platform.