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Coastal plain stratigraphy records tectonic, environmental, and human habitability changes related to sea-level drawdown, ‘Upolu, Sāmoa

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Abstract

Coastal plain stratigraphy is often over looked in paleo–sea-level reconstructions because carbonate sediments do not precisely constrain former sea level. Pacific Island sedimentology provides an invaluable record of geomorphic and environmental consequences of coastal evolution in response to changes in sea level and local tectonics. A series of coastal auger cores obtained from eastern ʻUpolu reveal a subsurface carbonate sand envelope predominately composed of coral and coralline algae derived from the reef framework. Coupling the sedimentological record with geophysical models of Holocene sea level, we identify a critical value (0.3–1.0 m) during the falling phase of the sea-level high stand (1899–2103 cal yr BP) that represents the transition from a transgressive to a regressive environment and initiates coastal progradation. Correlating the critical value with time, we observe nearly a millennium of coastal plain development is required before a small human population is established. Our findings support previous studies arguing that Sāmoa was colonized by small and isolated groups, as post–mid-Holocene drawdown in regional sea level produced coastal settings that were morphologically attractive for human settlement. As future sea level approaches mid-Holocene high stand values, lessons learned from Pacific Island sedimentological records may be useful in guiding future decisions related to coastal processes and habitat suitability.

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... The Mulifanua deposit containing Lapita pottery, lithics, and faunal remains dates to ca. 2750 cal BP and was discovered over 100 m offshore beneath a layer of beachrock during mechanical excavation for a car-ferry berth (Petchey, 1995(Petchey, , 2001Leach and Green, 1989). Additional geoarchaeological and geological studies have shown that 'Upolu is subsiding due to Savai'i island's lithospheric loading, and it is subsiding at a faster rate in the west near Mulifanua than in the east (Kane et al., 2017;Goodwin and Grossman, 2003), although possible tectonic influences on differential subsidence along a north-to southcoast gradient have not been investigated. ...
... The latter is a possibility given that there appears to be varying intensity of coastal use over time at nearby Manono, a small island offshore from Mulifanua (Sand et al., 2016). To test the terrestrial destruction hypothesis as an explanation for the lack of early terrestrial archaeological deposits, deeper excavations, chronological, sedimentological, and a micromorphological analyses (e.g., Kane et al., 2017) are required. Ideally, this work should focus on deposits near Core 1, the only core that approached the depth of the submarine Lapita deposit, and should use an engine-powered corer (e.g., vibra-corer) to recover sediments between the bottom depth of the auger cores and confirmed Lapita-age deposits. ...
... If the last 400 years are a guide, terrestrial destruction of deposits seems unlikely, even in this highly dynamic depositional environment, but the aforementioned tools, along with appropriate geoarchaeological analyses and dating will be required to evaluate this. Finally, along the eastern coastline of Aleipata, previous excavation and analysis of auger cores in Satitoa village indicated that the current coastal flat began to form ca. 2000 cal BP (Kane et al., 2017). The earliest cultural deposits on this landform are ca. ...
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Share Link, valid to 2 September: https://authors.elsevier.com/a/1ZOAI,rVDBRdpO The Remote Oceanic archipelagos from Vanuatu to Sāmoa were first occupied 3000 years ago by populations with Lapita pottery at over 100 colonization sites. In Sāmoa, however, the first millennium of settlement is comprised of only a few isolated archaeological sites, and only one with Lapita pottery. This unique archaeological record is typically explained as a result of isostatic subsidence that destroyed or displaced more numerous coastal colonization sites. Three additional hypotheses may account for this pattern. First, few coastal flats may have existed for settlement, limiting occupation of the archipelago. Second, terrestrial geological processes may have destroyed what were once more numerous sites. Third, the few early and isolated sites in Sāmoa may reflect a small population of colonists resulting from demographic processes, including wave-front population density, or the Allee effect. We conducted a preliminary examination of the first two alternative hypotheses through a programme of coring and excavation across three coastlines on 'Upolu island, Sāmoa. Sub-surface sediment data suggest both hypotheses may be valid explanations in different coastal settings. We propose additional research to test this possibility.
... As illustrated by Carter (1988), a robust body of literature documents the ecologic transition of these environments from the shoreline over geomorphic features (e.g., dunes and bluffs) landward. In fact, a relatively steady SLR rate over the last few thousand years is central to our modern coastal configuration, including the development of barrier islands and wetlands (e.g., Redfield, 1972;Field and Duane, 1976;Shennan and Horton, 2002), as well as settlement patterns (McGranahan et al., 2007;Liu et al., 2015;Kane et al., 2017). Because coastal land elevation is primarily governed by the substrate and/or underlying geology of the landscape, as well as being a prod-uct of the physical and biogeochemical processes acting on it, it serves as a central parameter in defining the distribution and configuration of ecosystems and their ability to evolve in response to processes driving change Kempeneers et al., 2009). ...
... In fact, a relatively steady SLR rate over the last few thousand years is central to our modern coastal configuration, including the development of barrier islands and wetlands (e.g., Redfield, 1972;Field and Duane, 1976;Shennan and Horton, 2002), as well as settlement patterns (McGranahan et al., 2007;Liu et al., 2015;Kane et al., 2017). Because coastal land elevation is primarily governed by the substrate and/or underlying geology of the landscape, as well as being a prod-uct of the physical and biogeochemical processes acting on it, it serves as a central parameter in defining the distribution and configuration of ecosystems and their ability to evolve in response to processes driving change Kempeneers et al., 2009). ...
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... Samples from these proveniences were chosen to provide estimates for the initial colonization of Tavua Island and to infer geomorphic response to sea-level retreat on Tavua. While coastal plain sediments cannot be used to establish precise relationships with past sea level, subsurface carbonates are valuable for determining the chronology of shoreline regression when paired with local and regional sea-level curves (following Kane et al., 2017). These chronometric determinations were used to create a Bayesian model in Oxcal 4.2 (Bronk Ramsey, 2009, 2013. ...
... Sample WK40062 (3294 ± 21 BP) is located below the initial Lapita deposit at Tavua at an elevation of approximately 1.75 m above MSL (±15 cm). While the dated coral clasts are not in growth position and therefore cannot provide a precise and reliable proxy for sea level, the vertical position, chronology, and sedimentological characteristic are generally consistent with the current understanding of mid-to late-Holocene sea level in the Fijian Islands and the South Pacific and its effect on the development of coastal plains (Kane et al., 2017;Allen et al., 2016). Based on the date of the sample and its position subordinate to the Lapita deposit, within a few centuries of 3181-2917 cal. ...
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The timing and choice of initial settlement location are examined on the small island of Tavua in Fiji’s Mamanuca Group. The mid- to late-Holocene sea-level retreat influenced the island’s coastal landforms through the acceleration of coastal progradation and the production of habitable land. Archaeological, sedimentological, and chronological data are integrated to better understand the island’s settlement and geomorphological history. These datasets are then compared with regional and modeled sea-level curves for Fiji in order to constrain the time period for the onset of coastal regression. The results indicate that Tavua was initially settled around 3000 years ago, within a few centuries of the formation of the coastal plain. Integrating archaeological, sedimentological, and sea-level datasets helps produce a more precise understanding of the relationship between sea-level change and the timing of settlement on small islands in Oceania.
... and the initiation of coastal plain development in ΄Upolu, Samoa from 2103 to 1899 cal yr BP(Kane et al., 2017).The marsh sediments above the basal carbonate sands are characterized by peat interbedded ...
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Over the past 200 years of written records, the Hawaiian Islands have experienced tens of tsunamis generated by earthquakes in the subduction zones of the Pacific ‘Ring of Fire’ (for example, Alaska–Aleutian, Kuril–Kamchatka, Chile and Japan). Mapping and dating anomalous beds of sand and silt deposited by tsunamis in low‐lying areas along Pacific coasts, even those distant from subduction zones, is critical for assessing tsunami hazard throughout the Pacific basin. This study searched for evidence of tsunami inundation using stratigraphic and sedimentological analyses of potential tsunami deposits beneath present and former Hawaiian wetlands, coastal lagoons, and river floodplains. Coastal wetland sites on the islands of Hawai΄i, Maui, O΄ahu and Kaua΄i were selected based on historical tsunami runup, numerical inundation modelling, proximity to sandy source sediments, degree of historical wetland disturbance, and breadth of prior geological and archaeological investigations. Sand beds containing marine calcareous sediment within peaty and/or muddy wetland deposits on the north and north‐eastern shores of Kaua΄i, O΄ahu and Hawai΄i were interpreted as tsunami deposits. At some sites, deposits of the 1946 and 1957 Aleutian tsunamis are analogues for deeper, older probable tsunami deposits. Radiocarbon‐based age models date sand beds from three sites to ca 700 to 500 cal yr BP, which overlaps ages for tsunami deposits in the eastern Aleutian Islands that record a local subduction zone earthquake (Witter et al., 2016, 2018). The overlapping modelled ages for tsunami deposits at the study sites support a plausible correlation with an eastern Aleutian earthquake source for a large prehistoric tsunami in the Hawaiian Islands. This article is protected by copyright. All rights reserved.
... In addition, tsunami deposits can vary greatly in their sedimentological characteristics and overall architecture due to different site and depositional conditions, as well as hydrodynamic differences in tsunami waves. On Hawai'i, both tsunamis and tropical storms have a high frequency and impact on deposition and erosion along shorelines (Kane et al., 2017). Even though tropical storms are capable of depositing massive sandy deposits, the inland extent and other characteristics of carbonate-rich sand layers (e.g. ...
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Far‐field tsunami deposits observed in the Kahana Valley, O'ahu, Hawai'i (USA), were investigated for their organic‐geochemical content. During short high‐energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid‐liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre‐date industrial activity in Hawaii by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input‐sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high‐resolution, multi‐proxy method for identifying tsunamis in the sedimentary record. This article is protected by copyright. All rights reserved.
... dunes and bluffs) landward. In fact, a relatively stable SLR rate over the last few thousand years is central to our modern coastal configuration, including the development of barrier islands and wetlands (e.g., Redfield, 5 1972;Field & Duane, 1976;Shennan & Horton, 2002), as well as settlement patterns (McGranahan et al., 2007;Liu et al., 2015;Kane et al., 2017). Elevation is a key parameter in defining the distribution and configuration of coastal ecosystems and their ability to evolve in response to processes driving change (Gesch, 2009;Kempeneers et al., 2009). ...
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Understanding land loss or resilience in response to sea-level rise (SLR) requires spatially extensive and continuous datasets to capture landscape variability. We investigate sensitivity and skill of a model that predicts dynamic response likelihood to SLR across the northeastern U.S. by exploring several data inputs and outcomes. Using elevation and land cover datasets, we determine where data error is likely, quantify its effect on predictions, and evaluate its influence on prediction confidence. Results show data error is concentrated in low-lying areas with little impact on prediction skill, as the inherent correlation between the datasets can be exploited to reduce data uncertainty using Bayesian inference. This suggests the approach may be extended to regions with limited data availability and/or poor quality. Furthermore, we verify that model sensitivity in these first-order landscape change assessments is well-matched to larger coastal process uncertainties, for which process-based models are important complements to further reduce uncertainty.
... This interpretation more recently has been challenged [34]. Substantive research into shoreline geomorphology [35] suggest there to be highly restricted coastal plains on most islands while modern shoreline features did not occur until after 2000 BP. The Samoan landscape seems ill-suited to settlement expansion, with colonization limited to but a few isolated groups [36]. ...
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The September 29th 2009 tsunami caused widespread coastal modification within the islands of Samoa and northern Tonga in the South Pacific. Preliminary measurements indicate maximum runup values of around 17m (Okal et al., 2010) and shore-normal inundation distances of up to ~620m (Jaffe et al., 2010). Geological field reconnaissance studies were conducted as part of an UNESCO-IOC International Tsunami Survey Team survey within three weeks of the event in order to document the erosion, transport, and deposition of sediment by the tsunami. Data collected included: a) general morphology and geological characteristics of the coast, b) evidence of tsunami flow (inundation, flow depth and direction, wave height and runup), c) surficial and subsurface sediment samples including deposit thickness and extent, d) topographic mapping, and e) boulder size and location measurements. Four main types of sedimentary deposits were identified: a) gravel fields consisting mostly of isolated cobbles and boulders, b) sand sheets from a few to ~25cm thick, c) piles of organic (mostly vegetation) and man-made material forming debris ramparts, and d) surface mud deposits that settled from suspension from standing water in the tsunami aftermath. Tsunami deposits within the reef system were not widespread, however, surficial changes to the reefs were observed.
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Two new formalisms are developed for the gravitationally self-consistent solution of sea level equation that governs the redistribution of the glacial melt water on a viscoelastic earth. The first formalism is a purely spectral technique based on an extension of Dahlen's (1976) theory for determining the equilibrium oceanic tide on an elastic planet. The second formalism, called 'pseudospectral', permits gravitationally self-consistent solutions of the sea level equation to much higher degree and order, making it possible to determine very accurately the low degree signal in the ocean water redistribution. As a consequence, the pseudospectral formalism will be especially useful in the gravitationally self-consistent analysis of present-day secular variations in the low-degree zonal harmonics of the earth's geoid.
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The South Pacific convergence zone (SPCZ) is the Southern Hemisphere's most expansive and persistent rain band, extending from the equatorial western Pacific Ocean southeastward towards French Polynesia. Owing to its strong rainfall gradient, a small displacement in the position of the SPCZ causes drastic changes to hydroclimatic conditions and the frequency of extreme weather events--such as droughts, floods and tropical cyclones--experienced by vulnerable island countries in the region. The SPCZ position varies from its climatological mean location with the El Niño/Southern Oscillation (ENSO), moving a few degrees northward during moderate El Niño events and southward during La Niña events. During strong El Niño events, however, the SPCZ undergoes an extreme swing--by up to ten degrees of latitude toward the Equator--and collapses to a more zonally oriented structure with commensurately severe weather impacts. Understanding changes in the characteristics of the SPCZ in a changing climate is therefore of broad scientific and socioeconomic interest. Here we present climate modelling evidence for a near doubling in the occurrences of zonal SPCZ events between the periods 1891-1990 and 1991-2090 in response to greenhouse warming, even in the absence of a consensus on how ENSO will change. We estimate the increase in zonal SPCZ events from an aggregation of the climate models in the Coupled Model Intercomparison Project phases 3 and 5 (CMIP3 and CMIP5) multi-model database that are able to simulate such events. The change is caused by a projected enhanced equatorial warming in the Pacific and may lead to more frequent occurrences of extreme events across the Pacific island nations most affected by zonal SPCZ events.
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Unconsolidated carbonate sands and cobbles on Kapapa Island, windward Oahu, are 1.4-2.8 (± 0.25) m above present mean sea level (msl). Agreeing with Stearns (1935), we interpret the deposit to be a fossil beach or shoreline representing a highstand of relative sea level during middle to late Holocene time. Calibrated radiocarbon dates of coral and mollusc samples, and a consideration of the effect of wave energy setup, indicate that paleo-msl was at least 1.6 (± 0.45) m above present msl prior to 3889-3665 cal. yr B.P., possibly as early as 5532-5294 cal. yr B.P., and lasted until at least 2239-1940 cal. yr B.P. Hence, the main phase of deposition on Kapapa Island lasted a minimum of c. 1400 yr and possibly as long as c. 3400 yr. No modern samples have been recovered from the fossil beach on Kapapa Island, and samples from potential source sites offshore of the island show modern ages, indicating that sediments on the island are not deposited by modern-era storm and tsunami overwash. Because antecedent sediments are uncommon offshore but common on the island, deposition must have been time-transgressive rather than related to a single event. Radiocarbon ages of coral and mollusc clasts from a breccia lining an emerged (1.4 ± 0.25 m msl) intertidal notch, cut into emerged coralline-algal carbonate of presumed last interglacial age, on south Mokulua Island (15 km to the southeast of Kapapa Island) correlate to the history recorded on Kapapa Island. Calibrated ages range from 2755-2671 to 3757-3580 cal. yr B.P. (averaging c. 3100 cal. yr B.P.) suggesting that a higher than present sea level formed the notch prior to 3757-3580 cal. yr B.P. A storm or tsunami origin for the features on Kapapa and south Mokulua islands is highly unlikely. Their age and elevation indicate, instead, a history of higher relative sea level (and subsequent fall) on windward Oahu during the middle to late Holocene. This history is consistent with geophysical models of postglacial geoid subsidence over the equatorial ocean first predicted by Walcott (1972) and later refined by Clark et al. (1978) and Mitrovica and Peltier (1991).
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There has been geographical variation in sea level since rapid postglacial melting of polar ice ceased similar to 6 k.y. ago, reflecting isostatic adjustments of Earth and ocean surfaces to past (and ongoing) redistribution of ice and water loads. A new data set of over 100 fossil microatolls from Christmas (Kiritimati) Island provides a Holocene sea-level record of unparalleled continuity. Living reef-flat corals grow up to a low-tide level. Adjacent fossil microatolls, long-lived Porites corals up to several meters in diameter, occur at similar elevations (+/-0.1 m), and extensive fossil microatolls in the island interior are at consistent elevations within each population. Collectively, they comprise an almost continuous sequence spanning the past 5 k.y., indicating that, locally, sea level has been within 0.25 m of its present position, and precluding global sea-level oscillations of one or more meters inferred from less stable locations, or using other sea-level indicators. This mid-Pacific atoll is tectonically stable and far from former ice sheets. The precisely surveyed and radiometrically dated microatolls indicate that sea level has not experienced significant oscillations, in accordance with geophysical modeling, which implies that the eustatic contribution from past ice melt and the isostatic adjustment of the ocean floor to loading largely cancel each other at this site.
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Because it lies at the intersection of Earth's solid, liquid, and gaseous components, sea level links the dynamics of the fluid part of the planet with those of the solid part of the planet. Here, I review the past quarter century of sea-level research and show that the solid components of Earth exert a controlling influence on the amplitudes and patterns of sea-level change across time scales ranging from years to billions of years. On the shortest time scales (10(0)-10(2) yr), elastic deformation causes the ground surface to uplift instantaneously near deglaciating areas while the sea surface depresses due to diminished gravitational attraction. This produces spatial variations in rates of relative sea-level change (measured relative to the ground surface), with amplitudes of several millimeters per year. These sea-level "fingerprints" are characteristic of (and may help identify) the deglaciation source, and they can have significant societal importance because they will control rates of coastal inundation in the coming century. On time scales of 10(3)-10(5) yr, the solid Earth's time-dependent viscous response to deglaciation also produces spatially varying patterns of relative sea-level change, with centimeters-per-year amplitude, that depend on the time-history of deglaciation. These variations, on average, cause net seafloor subsidence and therefore global sea-level drop. On time scales of 10(6)-10(8) yr, convection of Earth's mantle also supports long-wavelength topographic relief that changes as continents migrate and mantle flow patterns evolve. This changing "dynamic topography" causes meters per millions of years of relative sea-level change, even along seemingly "stable" continental margins, which affects all stratigraphic records of Phanerozoic sea level. Nevertheless, several such records indicate sea-level drop of similar to 230 m since a mid-Cretaceous highstand, when continental transgressions were occurring worldwide. This global drop results from several factors that combine to expand the "container" volume of the ocean basins. Most importantly, ridge volume decrease since the mid-Cretaceous, caused by an similar to 50% slow-down in seafloor spreading rate documented by tectonic reconstructions, explains similar to 250 m of sea-level fall. These tectonic changes have been accompanied by a decline in the volume of volcanic edifices on Pacific seafloor, continental convergence above the former Tethys Ocean, and the onset of glaciation, which dropped sea level by similar to 40, similar to 20, and similar to 60 m, respectively. These drops were approximately offset by an increase in the volume of Atlantic sediments and net seafloor uplift by dynamic topography, which each elevated sea level by similar to 60 m. Across supercontinental cycles, expected variations in ridge volume, dynamic topography, and continental compression together roughly explain observed sea-level variations throughout Pangean assembly and dispersal. On the longest time scales (10(9) yr), sea level may change as ocean water is exchanged with reservoirs stored by hydrous minerals within the mantle interior. Mantle cooling during the past few billion years may have accelerated drainage down subduction zones and decreased degassing at mid-ocean ridges, causing enough sea-level drop to impact the Phanerozoic sea-level budget. For all time scales, future advances in the study of sea-level change will result from improved observations of lateral variations in sea-level change, and a better understanding of the solid Earth deformations that cause them.
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Today’s understanding of sea-level change developed through a combination of process-based physical modelling and observational data. Observational data of sea-level change derives from coral reefs in the far-field of the former ice sheets where a geographically variable relative sea-level signal is expected as a response of the earth to ocean loading. Given this variability and the limited geographical distribution of coral reefs, there is a need to explore other, non-coral based sea-level markers to further understand sea-level change and and, for example, to ‘fingerprint’ melt-water. Here, we present beachrock as a coastal deposit suitable for relative sea-level (RSL) observations in the far-field. Beachrock is an intertidal deposit forming in the zone where carbonate saturated meteoric and marine water mix and pCO2 decreases. We provide the conceptual framework for beachrock analysis and describe techniques suitable for analysing and dating the deposit. The approach is standardised by outlining the sediment characteristics in terms of RSL indicative meaning and indicative range, and is tested against published data. A study conducted on coasts of the Mediterranean Sea exemplifies the utility of beachrock for RSL reconstruction. It is shown that the precision of the reconstruction is derived from the combined uncertainty of age and tidal amplitude or tidal range. The uncertainty can be reduced to half the tidal amplitude when a deposit can be ascribed to the upper (or lower) intertidal zone. Beachrock-based data benefit from the lack of non-quantifiable error terms such as post-depositional compaction due to the instantaneous formation and high preservation potential of the deposit. This underlines the high precision of beachrock-based RSL reconstruction, which is a prime requirement for testing and extending coral-based records.
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Understanding the history of relative Holocene sea levels on Pacific islands is important for constraining fundamental geodynamic theories, interpreting the environments of early human occupation sites, and forecasting future environmental conditions on the islands. An observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels, hence standing at higher elevations than modern counterparts. Emergent paleoshoreline notches in limestone seacliffs record paleo-high-tide levels and emergent paleoreef flats record paleo-low-tide levels, whereas emergent paleobeachrock locally records paleo-intertidal levels. Both paleonotches and paleoreefs occur along the coasts of high-standing islands exposing volcanic bedrock and uplifted reef complexes, but low-lying coralline atolls lack sufficient relief to preserve paleonotches. Controls on relative Holocene sea level include global eustatic and regional hydro-isostatic changes in ambient sea level relative to island landmasses, and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism, by flexure of the lithosphere under the load of growing volcanic edifices (Hawaii, Samoa, Society Islands), by arching of the lithosphere over trench forebulges (Loyalty Islands, Niue, Bellona–Rennell), and by tectonism within forearc belts between active volcanic chains and trenches (Mariana Islands, Tonga, Vanuatu). The dominant pattern of relative sea-level change, where not overprinted by local tectonism or lithospheric flexure, was a uniform early Holocene rise in eustatic sea level followed by a regionally variable late Holocene hydro-isostatic drawdown in sea level. The resultant was a mid-Holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical Pacific Ocean. The earliest human migrations into intra-oceanic island groups of both the northwestern and southwestern Pacific Ocean followed close on the heels of the mid-Holocene sea-level highstand, and took advantage of newly attractive coastal environments engendered by sea-level drawdown. The effects of the mid-Holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence.
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Coastal beach ridges have been favored locales for human habitations on Pacific islands since initial human settlement. Their smooth crests shaped by surf action form sandy substrates that differ in morphology from eolian dunes. Island beach ridges that were accreted to island cores during post-mid-Holocene drawdown in regional hydro-isostatic sea level include coastal spits that partially enclose backbeach wetlands, tombolos that tie previously separate islets together, and cheniers that form elongate elevated tracts on accretionary coastal plains. Some beach ridges were accreted directly to the flanks of bedrock islands, but others grew at varying distances from ancestral bedrock shorelines to trap remnants of offshore lagoons as lakes or swamps and marshes between the beach ridges and exposures of island bedrock. Composite chenier plains formed by the successive lateral accretion of multiple beach ridges over time are prominent locally as broad coastal lowlands. Longshore sand transport driven by prevailing trade winds influences beach ridge evolution but beach dynamics reflected by beach sand textures confirm the dominance of onshore sand movement from wave attack for the morphology of the beach ridges.
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During the Holocene (the last 11,500 calendar years) Earth's climate has been highly variable, not stable. Although the dramatic climate disruptions of the last glacial period have received considerable attention, relatively little has been directed toward the climate variability of the Holocene. Examination of nearly fifty, globally distributed paleoclimate records reveals that as many as six periods of rapid climate change (RCC) ca. 9-8, 6-5, 4.2-3.8, 3.5-2.5, 1.2-1.0, and beginning at 0.6 ka BP have occurred. All but the ca. 9-8 ka RCC and the most recent RCC are characterized in general by bipolar cooling, intensification of atmospheric circulation in the high latitudes, and increased aridity at low latitudes. The RCC since 0.6 ka features bipolar cooling and both humidity and aridity in the low latitudes. This interval appears to be more complex than the classic cool poles, dry tropics pattern that typified the Pleistocene and most of the Holocene RCCs. Several RCCs coincide with major disruptions of civilizations, underscoring the significance of Holocene climate variability. Of all potential climate forcing mechanisms considered, solar variability superimposed on long term changes in insolation seems to be most important for all of the RCCs except perhaps the ones ca. 9-8 ka and 4.2-3.8 ka.
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El Niño events are a prominent feature of climate variability with global climatic impacts. The 1997/98 episode, often referred to as `the climate event of the twentieth century', and the 1982/83 extreme El Niño, featured a pronounced eastward extension of the west Pacific warm pool and development of atmospheric convection, and hence a huge rainfall increase, in the usually cold and dry equatorial eastern Pacific. Such a massive reorganization of atmospheric convection, which we define as an extreme El Niño, severely disrupted global weather patterns, affecting ecosystems, agriculture, tropical cyclones, drought, bushfires, floods and other extreme weather events worldwide. Potential future changes in such extreme El Niño occurrences could have profound socio-economic consequences. Here we present climate modelling evidence for a doubling in the occurrences in the future in response to greenhouse warming. We estimate the change by aggregating results from climate models in the Coupled Model Intercomparison Project phases 3 (CMIP3; ref. ) and 5 (CMIP5; ref. ) multi-model databases, and a perturbed physics ensemble. The increased frequency arises from a projected surface warming over the eastern equatorial Pacific that occurs faster than in the surrounding ocean waters, facilitating more occurrences of atmospheric convection in the eastern equatorial region.
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Fossil coral reefs along the northeastern coast of St Croix in the Caribbean Sea provide an 8000 year record of dated and interpreted Holocene sea-level change. We herein compare this record with the predictions of models of glacio-hydro-isostatic adjustment for St Croix and for additional sites at similar latitudes in the Greater and Lesser Antilles region. RSL predictions are based upon the model ICE-5G (VM2), and with a modified model ICE-6G (VM5A), both including and excluding the influence of rotational feedback. Misfits between the modeled sea levels and the local geologic data are most apparent for models without rotational feedback, particularly in the prediction of a +2 to +4 m unsupported mid-Holocene misfit at ∼4 ka, as well as a small-amplitude highstand that extends from 2.5 to 1.5 ka. Incorporation of the influence of rotational feedback provides the best fit to the data, largely eliminating the unsupported mid-Holocene misfit between the data field and the sea-level histories predicted by the models without rotational feedback, and fitting data older than 5 kyr more closely than a previously published latitudinally-averaged sea-level curve for the western Atlantic. The St Croix data therefore demonstrate that rotational influence extends at least 27° further south from its 45° N mid-latitude extremum along the US east coast, and identifies tropical latitudes as influenced by proglacial forebulge collapse. Implications for reef-based sea-level reconstruction include the ability to accurately model sea levels at specific tropical sites with partial Holocene chronologies using the ICE-6G (VM5A) model with rotational feedback. Latitudinally-averaged sea-level curves are therefore of limited use in understanding the relative importance of contributing physical influences on postglacial sea-level history.
Article
A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.
Article
Theoretical approaches to computing gravitationally self-consistent sea-level changes in consequence of ice growth and ablation are comprised of two parts. The first is a mapping between variations in global sea level and changes in ocean height (required to define the surface load), and the second is a method for computing global sea-level change arising from an arbitrary surface loading. In Mitrovica & Milne (2003) (Paper I) we described a new, generalized mapping between sea-level change and ocean height that takes exact account of the evolution of shorelines associated with both transgression and regression cycles and time-dependent marine-based ice margins. The theory is valid for any earth model. In this paper we extend our previous work in three ways. First, we derive an efficient, iterative numerical algorithm for solving the generalized sea-level equation. Secondly, we consider a special case of the new sea-level theory involving spherically symmetric earth models. Specifically, we combine our iterative numerical formulation with viscoelastic Love number theory to derive an extended pseudo-spectral algorithm for solving the new sea-level equation. This algorithm represents an extension of earlier methods developed for the fixed-shoreline case to precisely incorporate shoreline migration processes. Finally, using this special case, we quantitatively assess errors incurred in previous efforts to extend the traditional (fixed shoreline) sea-level equation of Farrell & Clark (1976) to treat time-dependent shorelines. We find that the approximations adopted by Johnston (1993) and Milne (1998) to treat transgression and regression at shorelines introduce negligible (~1 per cent) error into predictions of post-glacial relative sea-level histories. In contrast, the errors associated with the Peltier (1994) sea-level equation are an order of magnitude larger, and comparable to the error incurred using the traditional sea-level theory. Furthermore, our numerical tests verify the high accuracy of the Milne (1998) approximation for treating the influence of grounded, marine-based ice.
Article
Atoll islands are areas of low, flat land, and the sustainability of habitable land in such environments is sensitive to even slight changes in sea level. The rise in sea level projected to occur during this century may lead to the submergence of atoll islands and the widespread loss of habitable land. However, the actual time sequence of past sea level change, island emergence events, and human settlement of newly emerged islands remain poorly constrained. Our excavation survey, combined with calibrated radiocarbon age dates, at Majuro Atoll, Marshall Islands, central Pacific, reveals that emergence of the island, triggered by a fall in sea level, was quickly followed by human settlement. The elevation of the central body of the island exceeded high water level at 2000 years ago, and the complete formation of the island occurred within an interval of 100 years. The island was colonized by people shortly after emergence, at 2000 years ago, prior to the establishment of dense vegetation, and has been continuously settled since that time. Habitable land was created by a fall in sea level, and any future rise in sea level will have a reverse effect, resulting in a loss of habitable land on atoll islands.
Article
New data from an emerged coastal bench and associated fossil beach on Kapapa Island (Oahu), Hawaii, preserve a detailed history of middle to late Holocene sea level. These include 29 new calibrated radiocarbon ages and elevations indicating mean sea level reached a maximum position of 2.00 ± 0.35 m ca. 3500 yr B.P. These results correlate with additional evidence from Hawaii and other Pacific islands and provide constraints on Oahu's long-term uplift rate (0.03 0.07 mm/yr), previously based solely on Pleistocene age shorelines. Our sea-level reconstruction is consistent with geophysical model predictions of Earth's geoid response to the last deglaciation and with observations of increased Antarctic ice volume during the late Holocene.
Article
The generation and composition of carbonate sediment within tropical carbonate settings is controlled by the skeletal production of CaCO3 in conjunction with physical, biological and chemical processes which act to break down and disperse skeletal remains. Using the results of detailed tumbling barrel experiments, this paper discusses the role the physical durability of common constituents of reef sediment plays in the composition of coral reef deposits. The durability of the skeletal remains of six reef sediment constituents was determined experimentally using tumbling barrels. Results indicate that constituent durability varies considerably amongst common reef sediment constituents. Calcareous algae Halimeda was the least durable constituent tested, followed by larger benthic foraminifera Baculogypsina sphaerulata and Marginopora vertebralis and the pelecypod Fragum fragum. Two species of branching coral (Acropora sarmentosa and Acropora nasuta) were found to be the most resistant to physical destruction. These findings provide increased power to interpret reef and island deposits and the potential role skeletal durability plays in the retention of constituent skeletons within coral reef associated sedimentary systems.
Article
A 25.8-m-thick sedimentary section containing coral fragments occurs directly below a surface lava flow (the -1340 year old Panaewa lava flow) at the Hilo drill hole. Ten coral samples from this section dated by accelerator mass spectrometry.(AMS) radiocarbon and five by thermal infrared multispectral scanner (TIMS) 23øTh/U methods show good agreement. The calcareous unit is 9790 years old at the bottom and 1690 years old at the top and was deposited in a shallow lagoon behind an actively growing reef. This sedimentary unit is underlain by a 34-m-thick lava flow which in turn overlies a thin volcaniclastic silt with coral fragments that yield a single 4C date of 10,340 years. The age-depth relations of the dated samples can be compared with proposed eustatic sea level curves after allowance for island subsidence is taken. Island subsidence averages 2.2 mm/yr for the last 47 years based on measurements from a tide gage near the drill hole or 2.5- 2.6 mm/yr for the last 500,000 years based on the ages and depths of a series of drowned coral reefs offshore from west Hawaii. The age-depth measurements of coral fragments are more consistent with eustatic sea levels as determined by coral dating at Barbados and Albrolhos Islands than those based on oxygen isotopic data from deep sea cores. The Panaewa lava flow entered a lagoon underlain by coral debris and covered the drill site with 30.9 m of lava of which 11 m was above sea level. This surface has now subsided to 4.2 m above sea level, but it demonstrates how a modern lava flow entering Hilo Bay would not only change the coastline but could extensively modify the offshore shelf.
Article
Microatolls are discoid corals with flat upper surfaces that develop when upward coral growth is constrained by exposure at low tide. They have been widely viewed as important and relatively precise indicators of modern and paleo sea-level positions because of their sensitivity to the water/air interface, though this has rarely been directly established by survey to a precise datum. This study involved an accurate survey of 282 microatolls growing in a range of intertidal environments (open reef flats, interisland passages, lagoons) across the Cocos (Keeling) Islands, a mid-ocean atoll in the Indian Ocean. The upper surface elevation of modern microatolls was found to vary through more than 40cm across the environments in which these corals occur on Cocos, representing more than 30% of the spring tide range. Much of the variation reflects subtle ponding at low water levels across extensive low-gradient reef surfaces associated with interisland passages. Microatolls in open reef flat habitats are typically constrained within the narrowest and most consistent elevation range around the atoll. The environment in which individual fossil microatolls occurred when they were alive is significant in reconstructing former sea levels.
Article
Radiocarbon dating of tests of Amphistegina spp. from the surface sand of Hawaiian beaches has revealed ages up to 1500 years or more. Preservation is related to age: the residual umbos are older than the intact tests. The antiquity of some of the tests suggests that predictions of a sand turnover rate of less than 100 years time based on Amphistegina productivity studies are in error and that the majority of tests produced in nearshore environments of Hawaii do not accumulate on the beach.
Article
The 1973 discovery of an underwater archaeological site during dredging for a ferry landing at Mulifanua on Upolu raised important unanswered questions about the prehistory of Samoa, particularly the evolution of Holocene shorelines and relative local sea levels. A cultural horizon yielding Lapita potsherds, the only decorated Lapita assemblage yet found in Samoa and dating to ca. 2.8 ka, lies at a depth of 2.25 m below modern sea level beneath a capping of cemented paleobeachrock. With fluctuating hydro-isostatic sea level taken into account, the sherd occurrence implies subsidence of a former coastline by ca. 4 m at a mean rate of 1.4 mm/yr. Shoreline features on both Upolu and nearby Savai'i are fully compatible with bulk Holocene subsidence. We attribute the observed subsidence to downflexure of the lithosphere from volcano loading centered on the Savai'i locus of historic volcanism, and conclude that any other Lapita sites that may exist in Samoa have subsided by a comparable amount. Although the Samoan linear volcanic chain resembles other Pacific hotspot tracks where active volcano loading is confined to their southeastern ends, the most voluminous Holocene eruptions in Samoa have occurred on Savai'i at the northwestern end of the exposed island chain. Samoan volcanism has evidently been influenced by lateral flexure of the Pacific plate as it moves past the northern extension of the Tonga subduction zone, and the active volcanism is apparently controlled by a longitudinal rift, which transects both Upolu and Savai'i and is superimposed upon older volcanic edifices that may record earlier hotspot volcanism. Early archaeological sites in American Samoa display a variable record of subsidence and possible uplift apparently related to volcano loading in the Manu'a Islands and possibly to the location of Tutuila in a position to be affected by uparching of lithosphere between downflexures beneath more active volcanic islands to the east and west. Bulk subsidence of Upolu and Savai'i may be the fundamental cause of damaging coastal erosion in modern Samoa. © 1998 John Wiley & Sons, Inc.
Article
Fluvial, marine, and mixed fluvial-marine deposition on the coastal plain of Hanalei Bay on the north shore of Kauai, Hawaii, records a middle- to late-Holocene fall of relative sea level. Radiocarbon dating of the regression boundary preserved in the stratigraphy of the coastal plain documents a seaward shift of the shoreline beginning at least 4800–4580 cal yr B.P. and continuing until at least 2160–1940 cal yr B.P. Marine sands stranded in the backshore and coastal plain environment are buried by fluvial floodplain and channel sands, silts, and muds. In places, erosion at the regression contact exposed older marine sands thus increasing the hiatus at the regression disconformity. The shoreline regression is best explained as the result of a fall in relative sea level. The age and elevation of the cored regression boundary at sites that have not been influenced by erosion are consistent with a middle- to late-Holocene highstand of relative sea level as predicted by geophysical models of whole Earth deformation related to deglaciation.
Article
The Waimanalo Formation (limestone) of Oahu has been correlated with the last interglacial period based on U-series dating of corals by T.-L. Ku and colleagues. The limestone consists of growth-position corals and overlying coral conglomerate. An apparent bimodal distribution of ages for the growth-position corals (mean age = 133 ka) and the overlying coral conglomerate (mean age = 119 ka) has been interpreted to represent two distinct high stands of sea that occurred within the last interglacial period. Both growth-position corals and overlying, conglomerate coral occur in an outcrop east of Kaena Point and consist mainly of Pocillopora and Porites. U-series ages of growth-position corals that show closed-system conditions are 120 ± 3 ka and 127 ± 4 ka; overlying conglomerate corals have U-series ages that range from 120 ± 3 ka to 138 ± 4 ka. At Kahe Point, conglomerate corals have ages of 120 ± 3 ka and 134 ± 4 ka. These data show that the growth position corals are not systematically older than the conglomerate corals; thus, there is no evidence for two distinct high stands of sea.Waimanalo deposits at Kahe Point and Mokapu Point (new U-series ages of 134 ± 4 ka and 127 ± 3 ka) have beach deposits as high as 12.5 m and, at Mokapu Point, growth-position corals as high as 8.5 m. A last-interglacial sea-level stand of +8.5 to +12.5 m conflicts with estimates of +6 m from a number of tectonically stable coastlines and islands in the western Atlantic Ocean. We infer, therefore, that Oahu may be undergoing uplift at a low rate. This uplift may be due to compensatory lithospheric flexure, because the island of Hawaii has been subsiding throughout much of the Quaternary from volcanic loading. Because of this possible uplift, Oahu and islands like it elsewhere in the Pacific cannot be used as reference points for sea level during the last interglacial period.
Article
The coral reef bordering the coastline of Samoa affected by the 29 September 2009 tsunami provides a variety of ecosystem services — from nurseries for fisheries and inshore source of food for local communities, to aesthetics for tourists, and the width of the lagoon may have been a factor in reducing the onshore wave height. To understand the complex interactions between the onshore human population and the offshore coral, we formed an interdisciplinary survey team to document the effects the tsunami had on the nearshore coral reef, and how these changes might affect local inhabitants. The scale of reef damage varied from severe, where piles of freshly-killed coral fragments and mortality were present, to areas that exhibited little impact, despite being overrun by the tsunami. We found that many coral colonies were impacted by tsunami-entrained coral debris, which had been ripped up and deposited on the fore reef by repeated cyclones and storm waves. In other places, large surface area tabular coral sustained damage as the tsunami velocity increased as it was funneled through channels. Areas that lacked debris entrained by the waves as well as areas in the lee of islands came through relatively unscathed, with the exception of the delicate corals that lived on a sandy substrate. In the lagoon on the south coast with its steep topography, coral colonies were damaged by tsunami-generated debris from onshore entrained in the backwash. Despite the potential for severe tsunami-related damage, there were no noticeable decreases in live coral cover between successive surveys at two locations, although algal cover was higher with the increased nutrients mobilized by the tsunami. While there was an immediate decrease in fish takes in the month following the tsunami, when supporting services were likely impacted, both volume and income have rapidly increased to pre-tsunami levels. Long-term monitoring should be implemented to determine if nursery services were affected.
Article
Multi-proxy studies of trench sediments from Futuna island, in the Wallis and Futuna archipelago, have produced a record of two palaeotsunamis dated to around 1860-2000 BP and c. 470 BP. One of the most useful proxies in this study has been the archaeological data associated with occupations that immediately underlie the deposits associated with both palaeotsunamis. The reworking of the archaeological material into the palaeotsunami deposits and the additional chronological control provided by artefacts have added value to interpretations based upon more conventional earth science proxies.
Article
Archaeological sites in the northern Ha'apai Group of central Tonga occur on small islands within the uplifted forearc belt of the Tonga-Kermadec arc-trench system. The present inland positions of occupation sites that probably once occupied coastal settings imply significant expansion of some island shorelines during late Holocene time (ca. 3250 B.P. to present). Geologic processes leading potentially to enlargement of the islands include continuing forearc uplift, eustatic or glacio-hydro-isostatic fall in sea level following a mid-Holocene highstand, and progressive accretion of beach ridges to island coasts, with or without changes in relative sea level. Radiometric dates for uplifted coral terraces in Tonga indicate that forearc uplift has been negligible during Holocene time. By contrast, theoretical considerations, regional analysis of shoreline indicators throughout the South Pacific, and limited empirical data from Tonga itself all imply that regional sea level has declined locally by 1–2 m since a mid-Holocene highstand (ca. 6000-3000 B.P.), which was a hydro-isostatic response to transfer of water mass from Pleistocene ice caps to the ocean basins. Emergence of originally coastal sites is thus expected since initial settlement of the islands by Lapita peoples. Accretionary coastal flats composed of multiple beach ridges are 250–500 m wide on favorable leeward shores and the flanks of sand cays, but some presently unknown proportion of this incremental island growth may have occurred prior to the post-mid-Holocene decline in relative sea level. Ash falls from tephra eruptions at Tongan volcanoes also modified island environments through Quaternary time. Evidence for significant change in the configuration and morphology of islands in Ha'apai during the period of human settlement highlights the need for systematic interdisciplinary archaeological and geological research in the study of Pacific prehistory. © 1994 John Wiley & Sons, Inc.
Article
Sandy deposits from the 29 September 2009 tsunami on the east coast of Upolu, Samoa were investigated to document their characteristics and used to apply an inverse sediment transport model to estimate tsunami flow speed. Sandy deposits 6 to 15 cm thick formed from ~ 25 to ~ 250 m inland. Sedimentary layers in the deposits, that are defined by vertical grain size variation and contacts, are interpreted to have formed during onshore runup of two waves. Deposits at 3 locations (100, 170, and 240 m inland) contained two layers that are predominately normally graded (~ 80%), but contained massive sections (~ 15%) and inversely graded sections (~ 5%) at their bases. About 75% of the total thickness of normally graded intervals exhibits a signature of sediment falling out of suspension at their top. This type of grading, termed suspension grading here, was first recognized in turbidity current deposits and is characterized by the entire distribution shifting finer upwards in a layer as high-settling velocity, coarser material deposits first and low-settling velocity finer material deposits last. The Jaffe and Gelfenbaum (2007) inverse sediment transport model was applied to intervals within layers that exhibited suspension grading to estimate tsunami flow speed and was able to reproduce the general trends of the observed suspension grading. A key unknown input in the modeling is the bottom roughness. For a bottom roughness parameterization using a Manning's n of 0.03 (equivalent to a z0 ~ 0.006 m for the observed flow depths of 2–3 m) flow speeds calculated for the 2 layers at the 3 locations were 3.8, 3.6, and 3.7 m/s (bottom layer/earlier wave) and 4.4, 4.4, and 4.1 m/s (top layer/later wave) at 100, 170, and 240 m inland, respectively. These estimates are consistent with the ~ 3–8 m/s tsunami flow speed from boulder transport calculations and result in Froude numbers of ~ 0.7–1.0 when maximum measured flow depths are used. Because the inverse model assumes the deposit was formed by sediment falling out of suspension care must be taken to model only intervals of the deposit exhibiting suspension grading. Including intervals deposited by either bedload or suspended load transport convergences result in higher, and sometimes unrealistic, tsunami flow speed estimates.
Article
Orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming. Ice-albedo, sea-level, and greenhouse-gas feedbacks, together with tropical warming from weak-ening winds in response to polar amplification of warming, caused regional-to-global (near-) synchronization of deglaciation. Effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly. Ice-sheet–linked changes in freshwater delivery to the North Atlantic, and possibly free oscillations in the climate system, forced millennial climate oscillations associated with changes in North Atlantic deep water (NADW) flow. The North Atlantic typically operates in one of three modes: modern, glacial, and Heinrich. Deglaciation occurred from a glacial-mode ocean that, in com-parison to modern, had shallower depth of penetration of NADW formed further south, causing strong northern cooling and the widespread cold, dry, and windy conditions associated with the glacial maximum and the cold phases of the mil-lennial Dansgaard-Oeschger oscillations. The glacial mode was punctuated by meltwater-forced Heinrich conditions that caused only small additional cooling at high northern latitudes, but greatly reduced the formation of NADW and triggered an oceanic "seesaw" that warmed some high-latitude southern regions centered in the South Atlantic.
Article
The ecosystems and economies of small island nation states and territories of the tropical southwest Pacific region are widely agreed to be among the most vulnerable to climate variability and weather extremes anywhere in the world. Tropical Cyclones (TCs) are capable of exacerbating existing hazards and those made more severe by climate change (e.g. local sea level rise). In order to properly understand TC impacts in this region, a comprehensive database of TC tracks is required. This work has collated TC best track data from forecast centres around the globe with the aim of producing a unified global best TC track dataset for the historical period. Data from the International Best Tracks for Climate Stewardship (IBTrACS) project information for the southwest Pacific (135°E–120°W, 5°–25°S) is built upon and included in this effort. We document the construction of an enhanced TC database for the southwest Pacific, the quality controls needed to construct the database, and discuss how it has enhanced the chronology of region-wide historical TC activity in light of newly discovered data. We suggest this enhanced dataset can be used in forthcoming climate and weather studies to better characterize the climatology and behaviour of TCs in the southwest Pacific. Copyright © 2011 Royal Meteorological Society