No abstract

No abstract

Tropical cyclones (TCs) are powerful agents of destruction, and understanding climatic controls on TC patterns is of great importance. Over timescales of seasons to several decades, relationships among TC track, frequency, intensity and basin-scale climate changes are well documented by instrumental records. Over centuries to millennia, climate-shift influence on TC regimes remains poorly constrained. To better understand these relationships, records from multiple locations of TC strikes spanning millennia with high temporal resolution are required, but such records are rare. Here we report on a highly detailed sedimentary proxy record of paleo-TC strikes from the Blue Hole of Lighthouse Reef, Belize. Our findings provide an important addition to other high-resolution records, which collectively demonstrate that shifts between active and inactive TC regimes have occurred contemporaneously with shifts hemispheric-scale oceanic and atmospheric circulation patterns such as MDR SSTs and NAO mode, rather than with changes in local climate phenomena as has previously been suggested.

No abstract

Multibeam maps and high-resolution seismic images from the Maldives reveal that a late Miocene to early Pliocene partial drowning of the platform was linked to strong sea-bottom currents. In the upper Miocene to Holocene, currents shaped the drowned banks, the current moats along the bank edges, and the submarine dune fi elds. Bottom currents in the Maldives are driven by the monsoon. It is proposed that the onset and the intensifi cation of the monsoon during the Neogene provoked platform drowning through injection of nutrients into surface waters. Since the late Miocene, topographically triggered nutrient upwelling and vigorous currents switched the Maldives atolls into an aggradational to backstepping mode, which is a growth pattern usually attributed to episodes of rising sea level.

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.

[1] Since Last Glacial Maximum (23-19 ka), Earth climate warming and deglaciation occurred in two major steps (Bølling-Allerød and Preboreal), interrupted by a short cooling interval referred to as the Younger Dryas (12.5-11.5 ka B.P.). In this study, three cores collected in the central part of Pandora Trough (Gulf of Papua) have been analyzed, and they reveal a detailed sedimentary pattern at millennial timescale. Siliciclastic turbidites disappeared during the Bølling-Allerød and Preboreal intervals to systematically reoccur during the Younger Dryas interval. Subsequent to the final disappearance of the siliciclastic turbidites a calciturbidite occurred during meltwater pulse 1B. The Holocene interval was characterized by a lack of siliciclastic turbidites, relatively high carbonate content, and fine bank-derived aragonitic sediment. The observed millennial timescale sedimentary variability can be explained by sea level fluctuations. During the Last Glacial Maximum, siliciclastic turbidites were numerous when the lowstand coastal system was located along the modern shelf edge. Although they did not occur during the intervals of maximum flooding of the shelf (during meltwater pulses 1A and 1B), siliciclastic turbidites reappear briefly during the Younger Dryas, an interval when sea level rise slowed, stopped, or perhaps even fell. The timing of the calciturbidite coincides with the first reflooding of Eastern Fields Reef, an atoll that remained exposed for most of the glacial stages.

The last glacial interval (oxygen isotope stages 4 2 or 75-12 ka) is considered overall as a sea-level lowstand relative to today (Holocene, i.e., since 12 ka) and the previous interglacial highstand stage 5 (130-75 ka). During this stage 4 2 interval, an earlier lagoon and barrier-reef system-established on the edge of the southern Belize shelf margin during the previous stage 5 interglacial highstand-was completely exposed. The exposed stage 5 lagoon area became a siliciclastic fluvial plain bounded on its eastern side by a ridge of karst topography created by the exposure of the stage 5 barrier reef.A high-resolution single-channel seismic survey in the northern part of the present Southern Shelf Lagoon off the coast of Belize was used to image two late Quaternary lowstand fluvial drainage systems consisting of a series of well-developed incised valleys, trending parallel to the strike of the exposed stage 5 barrier reef. The divide between both drainage systems, at the latitude of Dangriga (just south of 17"N), appears to be rooted on a topographic high composed of upper Tertiary deposits. South of Dangriga, the system of buried incised valleys drained the area toward the south and merged into Victoria Channel and other channels separating modem rhomboid reefs. North of Dangriga, the system of buried incised valleys drained into the single, partially filled, and deeply incised valley of the English Cay Channel. This valley cut through the exposed stage 5 barrierreef and back-barrier-reef system and linked the fluvial drainage system north of Dangriga to the Tumeffe Basin.The bathymetric contours at the mouth of the English Cay Channel protrude eastward across a thick, fanlike sedimentary body characterized by a series of stacked, prograding, and laterally shifting lobes. This sedimentary body is interpreted as a lowstand shelf-edge delta, similar in surface area to the modem highstand delta of the Belize River on which Belize City is located. Southerly directed currents partially reworked the lowstand shelfedge delta and created an elongated slope fan at the toe of slope of the exposed barrier reef.A model, tied to the established sea-level curve from 20 ka to the present, is developed to explain the lowstand geometry and morphology of the incised valleys and the sedimentary deposits in front of the modem Belize Barrier Reef. This model also describes, in a tentative time frame, the nature of the transgressive sedimentary fills in the valleys. Moreover, correlation of the single-channel high-resolution seismic grid along the westem margin of the Turneffe Basin with a multichannel seismic dip line shows that the lowstand shelf-edge delta at the mouth of the English Cay Channel and the slope fan, developed on the toe of the barrier reef farther south, correspond to the latest lowstand prograding sedimentary bodies that are mostly observed underlying the modem barrier-reef and back-barrier-reef system. The establishment and growth of carbonates over siliciclastic deposits-expressed in the colonization of patch r...