Sea-level changes have shaped the world's carbonate platform margins and continental shelves, leaving typical geomorphic imprints, such as drowned reef terraces. In this paper, we present the results ...of 112 scuba diving transects across seven different Maldivian atolls and one multibeam survey around Malé Island, the capital of Maldives. We report on the occurrence of drowned reef terraces down to 120 m depth. In total, we identified six levels of submerged terraces that we consider as indicative of periods of time with stable or slowly rising sea level that can be attributed either to deceleration of the last deglacial sea-level rise or to Late Quaternary sea-level highstands. We compare our dataset to the depth of reef terraces reported globally, and we discuss the reasons why common global submerged terrace levels are difficult to identify in the field record.
Although Late Cambrian microbial build‐ups were recognized in the Point Peak Member of the Wilberns Formation in Central Texas (USA) nearly 70 years ago, only a few studies focused specifically on ...the build‐ups themselves. This study focuses on the interpretation of the regional (15 measured sections described in literature representing an area of 8000 km2) and local (field and drone photogrammetry studies in a 25 km2 area from within south Mason County) microbial build‐up occurrence, describes their growth phases and details their interactions with the surrounding inter‐build‐up sediments. The study establishes the occurrence of microbial build‐ups in the lower and upper Point Peak members (the Point Peak Member is informally broken up into the lower Point Peak and the upper Point Peak members separated by Plectotrophia zone). The lower Point Peak Member consists of three <1 m thick microbial bioherms and biostrome units, in addition to heterolithic and skeletal/ooid grainstone and packstone beds. One, up to 14 m thick, microbial unit associated with inter‐build‐up skeletal and ooid grainstone and packstone beds, intercalated with mixed siliciclastic–carbonate silt beds, characterizes the upper Point Peak member. The microbial unit in the upper Point Peak member displays a three‐phase growth evolution, from an initial colonization phase on flat based, rip‐up clast lenses, to a second aggradation and lateral expansion phase, into a third well‐defined capping phase. The ultimate demise of the microbial build‐ups is interpreted to have been triggered by an increase of water turbidity caused by a sudden influx of fine siliciclastics. The lower Point Peak member represents inner ramp shallow subtidal and intertidal facies and the upper Point Peak member corresponds to mid‐outer ramp subtidal facies. Understanding the morphological architecture and depositional context of these features is of importance for identifying signatures of early life on Earth.
The depth of equatorial Pacific thermocline is diagnostic of the main modes of tropical climates. Past estimates of Pacific thermocline dynamics have been reconstructed either for the Last Glacial ...Maximum or on longer timescales at low resolution. Here we document a new high‐resolution set of reconstructed past sea surface and subsurface waters temperatures from the southwestern subequatorial Pacific, core MD05‐2930, in the Gulf of Papua, over the last 800 ka. We used two morphotypes of Globigerinoides ruber known to live at different water depths to reconstruct past stratification. We estimated calcification temperature of each morphotypes by Mg/Ca paleothermometry. Our subequatorial Pacific thermocline paleotemperature record indicates a response of the thermocline to both direct orbital forcing and glacial‐interglacial changes. Our stratification record shows a systematic shallower glacial thermocline, whereas sea surface temperatures are characterized by precessional forcing. The record is indicative of a progressive long‐term shoaling of the thermocline during the glacial stages during the late Pleistocene. The shoaling of the subequatorial Pacific thermocline is consistent with regional estimates. An enhanced South Pacific shallow overturning wind‐driven circulation could have driven this progressive shoaling. We speculate that this late Pleistocene glacial shoaling of the thermocline could be related to an increase in the amplitude of the obliquity.
Key Points
West equatorial Pacific thermocline shows a glacial‐interglacial beating
West equatorial Pacific thermocline shoals during glacials since 800 kyr
Glacial thermocline correlated to wind‐driven circulation
In 1842, Darwin identified three types of reefs: fringing reefs, which are directly attached to volcanic islands; barrier reefs, which are separated from volcanic islands by lagoons; and ring reefs, ...which enclose only a lagoon and are defined as atolls. Moreover, he linked these reef types through an evolutionary model in which an atoll is the logical end point of a subsiding volcanic edifice, as he was unaware of Quaternary glaciations. As an alternative, starting in the 1930s, several authors proposed the antecedent karst model; in this model, atolls formed as a direct interaction between subsidence and karst dissolution that occurred preferentially in the bank interiors rather than on their margins through exposure during glacial lowstands of sea level. Atolls then developed during deglacial reflooding of the glacial karstic morphologies by preferential stacked coral-reef growth along their margins. Here, a comprehensive new model is proposed, based on the antecedent karst model and well-established sea-level fluctuations during the last 5 million years, by demonstrating that most modern atolls from the Maldives Archipelago and from the tropical Pacific and southwest Indian Oceans are rooted on top of late Pliocene flat-topped banks. The volcanic basement, therefore, has had no influence on the late Quaternary development of these flat-topped banks into modern atolls. During the multiple glacial sea-level lowstands that intensified throughout the Quaternary, the tops of these banks were karstified; then, during each of the five mid-to-late Brunhes deglaciations, coral reoccupied their raised margins and grew vertically, keeping up with sea-level rise and creating the modern atolls.
Multichannel high‐resolution seismic and multibeam data were acquired from the Maldives‐isolated carbonate platform in the Indian Ocean for a detailed characterization of the Neogene bank ...architecture of this edifice. The goal of the research is to decipher the controlling factors of platform evolution, with a special emphasis on sea‐level changes and changes of the oceanic currents. The stacking pattern of Lower to Middle Miocene depositional sequences, with an evolution of a ramp geometry to a flat‐topped platform, reflects variations of accommodation, which here are proposed to be primarily governed by fluctuations of relative sea level. Easterly currents during this stage of bank growth controlled an asymmetric east‐directed progradation of the bank edge. During the late middle Miocene, this system was replaced by a twofold configuration of bank development. Bank growth continued synchronously with partial bank demise and associated sediment‐drift deposition. This turnover is attributed to the onset and/or intensification of the Indian monsoon and related upwelling and occurrence of currents, locally changing environmental conditions and impinging upon the carbonate system. Mega spill over lobes, shaped by reversing currents, formed as large‐scale prograding complexes, which have previously been interpreted as deposits formed during a forced regression. On a regional scale, a complex carbonate‐platform growth can occur, with a coexistence of bank‐margin progradation and aggradation, as well as partial drowning. It is further shown that a downward shift of clinoforms and offlapping geometries in carbonate platforms are not necessarily indicative for a sea‐level driven forced regression. Findings are expected to be applicable to other examples of Cenozoic platforms in the Indo‐Pacific region.
The upper Cambrian Wilberns Formation in central Texas records deposition on a low‐gradient shelf within a mixed carbonate–siliciclastic tidal‐flat system that changes offshore to subtidal shelf and ...open‐marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE–SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low‐frequency and intermediate‐frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea‐level fluctuation. Random facies stacking and the lack of metre‐scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms.
This paper characterizes a cyclic, spatially mixed carbonate–siliciclastic system developed in an epicratonic shelf strongly influenced by tides and storms. Microbialites formed both in the intertidal zone and in the offshore zone associated with skeletal and oolitic shoals dominantly controlled by accommodation space and currents. Chaotic facies stacking at the metre‐scale resulted from gaps in the record and episodic influence of storms.
Because the initial phase of barrier reef evolution is often buried under more recent phases of coralgal growth, the origins of modern barrier reefs have remained elusive. Direct observations on the ...nature of the substrate on top of which barrier reefs have developed are lacking, and simple questions about whether the substrate contributes to their overall linear morphology have remained unanswered. We present here a review dedicated to late-Quaternary shelf-edge deposition in tropical mixed siliciclastic-carbonate systems. These modern analogs are used to develop a quantitative understanding of shelf-edge barrier reef formation during different segments of relatively well-established sea-level cycles. The onset of rapid sea-level rise during early deglaciations, when siliciclastics were deposited along newly formed coasts at up-dip positions, provided opportune time windows for coralgal communities to establish themselves on top of maximum lowstand siliciclastic coastal deposits, such as beach ridges and lowstand shelf-edge deltas.
Coralgal reefs preserve the signatures of sea-level fluctuations over Earth's history, in particular since the Last Glacial Maximum 20,000 years ago, and are used in this study to indicate that ...punctuated sea-level rise events are more common than previously observed during the last deglaciation. Recognizing the nature of past sea-level rises (i.e., gradual or stepwise) during deglaciation is critical for informing models that predict future vertical behavior of global oceans. Here we present high-resolution bathymetric and seismic sonar data sets of 10 morphologically similar drowned reefs that grew during the last deglaciation and spread 120 km apart along the south Texas shelf edge. Herein, six commonly observed terrace levels are interpreted to be generated by several punctuated sea-level rise events forcing the reefs to shrink and backstep through time. These systematic and common terraces are interpreted to record punctuated sea-level rise events over timescales of decades to centuries during the last deglaciation, previously recognized only during the late Holocene.
Records of element ratios obtained from the Maldives Inner Sea sediments provide a detailed view on how the Indian Monsoon System has varied at high-resolution time scales. Here, we present records ...from International Ocean Discovery Program (IODP) Site U1471 based on a refined chronology through the past 550,000 years. The record's high resolution and a proper approach to set the chronology allowed us to reconstruct changes in the Indian Monsoon System on a scale of anomalies and to verify their relationships with established records from the East Asian Monsoon System. On the basis of Fe/sum and Fe/Si records, it can be demonstrated that the Asia continental aridity tracks sea-level changes, while the intensity of winter monsoon winds responds to changes in Northern Hemisphere summer insolation. Furthermore, the anomalies of continental aridity and intensity of winter monsoon winds at millennial-scale events exhibit power in the precession band, nearly in antiphase with Northern Hemisphere summer insolation. These observations indicate that the insolation drove the anomalies in the Indian Summer Monsoon. The good correspondence between our record and the East Asian monsoon anomaly records suggests the occurrence of anomalous widespread arid events in Asia.
The Great Barrier Reef (GBR) located along the northeastern margin of Australia is the largest coral reef system in the world. Modern climatic alterations are quickly changing the GBR ecosystem. To ...understand the implications of these changes it is important to reconstruct the geological history of GBR. Here we use geochemical and magnetic proxies to evaluate past climatic fluctuations and their consequences on sediment deposition along the GBR margin. IODP Expedition 325 – Hole M0058A, drilled on the uppermost slope at ca. 170 m water depth, reveals the depositional history of the GBR margin during the interval of MIS 7 to 5 and MIS 1. Magnetic and geochemical variations along the core section reveal detailed information on sediment accumulation and on the variations in terrigenous input in relation to sea-level fluctuations and climate change. Sea-level variations influenced margin deposition between MIS 7 and 6 impacting shoreline progradation/retrogradation and siliciclastic redistribution, resulting in a mixture of finer to coarser magnetic assemblages with no significant changes in terrigenous input. At the end of MIS 6 a decline in the deposition of carbonate sediments concomitant with the deposition of fine-grained magnetite-rich terrigenous sediments suggests an intensification of the monsoon in response to global warming trends. Arid periods over NE Australia were established after the glacial/interglacial transition (the MIS 6-5e) and at the middle Holocene (after the MIS 2–1 transition at ca. 7 ka), which favored dust deposition over the region. Enhanced dust fertilization subsequently promoted primary productivity at these intervals resulting in the presence of biogenic magnetite at Hole M0058A produced by magnetotactic bacteria.
•IODP Expedition 325 - Hole M0058A data reveal changing sedimentation accumulation in response to past climate changes.•A mixture of magnetic assemblages resulted from the redistribution of siliciclastic sediments.•Fine-grained magnetite indicates surface runoff, while biogenic magnetite is related to dust input.
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