Lars Reuning

Changes in Caribbean surface hydrography during the Pliocene shoaling of the Central American Seaway

Pliocene d18O records of shallow and deep dwelling planktonic foraminifers from the Caribbean (Ocean Drilling Program sites 999 and 1000), the tropical east Pacific (sites 1241 and 851), and the Atlantic (site 925, Ceara Rise, and site 1006, western Great Bahama Bank) were used to examine Atlantic-Caribbean-Pacific atmospheric and oceanic linkages associated with the progressive closure of the Central American Seaway (5.5–3 Ma). Comparisons indicate the development of an inner-Caribbean salinity gradient in the mixed layer and salinity changes on precessional periodicities after 4.4 Ma (site 1000), when the Pacific-Caribbean throughflow became significantly restricted. The high-amplitude variability in salinity is also observed at site 1006, monitoring the Caribbean outflow into the Atlantic. Comparisons of Caribbean and Atlantic planktonic d18O records suggest the North Atlantic subtropical gyre as a major source for high-salinity surface waters. Precession-induced variations in the volume transport of Pacific surface water masses through the Panamanian Seaway are considered a main factor to explain the Caribbean salinity minima. Results from a coupled climate model point to changes in the El Nin˜o–Southern Oscillation state as a potential trigger for changes in the amount of Pacific inflow into the Caribbean.

Foraminiferal Mg/Ca increase in the Caribbean during the Pliocene: Western Atlantic Warm Pool formation, salinity influence, or diagenetic overprint?

We constructed a high-resolution Mg/Ca record on the planktonic foraminifer Globigerinoides sacculifer in order to explore the change in sea surface temperature (SST) due to the shoaling of the Isthmus of Panama as well as the impact of secondary factors like diagenesis and large salinity fluctuations. The study covers the latest Miocene and the early Pliocene (5.6–3.9 Ma) and was combined with δ18O to isolate changes in sea surface salinity (SSS). Before 4.5 Ma, SSTMg/Ca and SSS show moderate fluctuations, indicating a free exchange of surface ocean water masses between the Pacific and the Atlantic. The increase in δ18O after 4.5 Ma represents increasing salinities in the Caribbean due to the progressive closure of the Panamanian Gateway. The increase in Mg/Ca toward values of maximum 7 mmol/mol suggests that secondary influences have played a significant role. Evidence of crystalline overgrowths on the foraminiferal tests in correlation with aragonite, Sr/Ca, and productivity cyclicities indicates a diagenetic overprint on the foraminiferal tests. Laser ablation inductively coupled plasma–mass spectrometry analyses, however, do not show significantly increased Mg/Ca ratios in the crystalline overgrowths, and neither do calculations based on pore water data conclusively result in significantly elevated Mg/Ca ratios in the crystalline overgrowths. Alternatively, the elevated Mg/Ca ratios might have been caused by salinity as the δ18O record of Site 1000 has been interpreted to represent large fluctuations in SSS, and cultivating experiments have shown an increase in Mg/Ca with increasing salinity. We conclude that the Mg/Ca record <4.5 Ma can only reliably be considered for paleoceanographical purposes when the minimum values, not showing any evidence of secondary influences, are used, resulting in a warming of central Caribbean surface water masses after 4.5 Ma of ∼2°C.

Sedimentation cycles and their diagenesis on the slope of a Miocene carbonate ramp (Bahamas, ODP Leg 166)

Numerous small-scale depositional cycles are present in the Miocene sediments of seismic sequence m along the margin of Great Bahama Bank (ODP Leg 166). These cycles consist of decimetre- to metre-scale alternations between light-grey and dark-grey/black wackestones/packstones. The light-grey layers are well cemented and nearly uncompacted. They contain planktonic and benthic foraminifera, and bioclasts. Bioturbation in these layers is moderate. The dark-grey wackestones/packstones are uncemented, strongly compacted and normally strongly bioturbated. The main components are planktonic foraminifera and fine-grained bioclasts. The dark layers are rich in aragonite and organic carbon and contain around 80% carbonate. The light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97.5%. Light δ13C and slightly negative δ18O values were observed in the dark uncemented layers while the cemented intervals show heavy δ13C values and slightly more positive δ18O values. The carbon isotope signal between the dark and the light layers shows variations of up to 1.45‰. Both δ13C and δ18O co-vary with carbonate content. The Formation MicroScanner images reflect the changes in carbonate mineralogy. The Natural Gamma-Ray Tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy. The internal stacking pattern of the cycles is closely tied to sea level. The dark layers are deposited during rising sea level, while the light layers reflect sediment production and export during highstand and falling sea level. These cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat-topped platforms. Most measured parameters suggest that primary sediment composition played an important role in the cementation process. The primary differences in composition were enhanced during shallow burial diagenesis. The initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite-rich layers leading to the formation of the light beds. Dissolution of metastable high-Mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite. Diagenesis was enhanced by the coarse grain size of the primary aragonite-rich layers. In contrast the more pelagic, dark calcite-rich layers were only slightly affected by diagenesis and could preserve their aragonite content. The uncemented layers subsequently were subject to strong mechanical compaction, whereas the cemented beds stayed nearly uncompacted.

Numerical modelling of the displacement and deformation of embedded rock bodies during salt tectonics: A case study from the South Oman Salt Basin

Abstract Large rock inclusions are embedded in many salt bodies and these respond to the movements of the salt in a variety of ways including displacement, folding and fracturing. One mode of salt tectonics is downbuilding, whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside. We investigate how the differential displacement of the top salt surface caused by downbuilding induces ductile salt flow and the associated deformation of brittle stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model, in combination with adaptive remeshing. The model set-up is constrained by observations from the South Oman Salt Basin, where large carbonate bodies encased in salt form substantial hydrocarbon plays. The model shows that, depending on the displacement of the top salt, the stringers can break very soon after the onset of salt tectonics and can deform in different ways. If extension along the inclusion dominates, stringers are broken by tensile fractures and boudinage at relatively shallow depth. Spacing of the boudin–bounding faults can be as close as 3–4 times the thickness of the stringer. In contrast, salt shortening along the inclusion may lead to folding or thrusting of stringers.

Sub-Milankovitch cycles in periplatform carbonates from the early Pliocene Great Bahama Bank

High-resolution bulk sediment (magnetic susceptibility and aragonite content) and δ18O records from two different planktonic foraminifera species were analyzed in an early Pliocene core interval from the Straits of Florida (Ocean Drilling Program site 1006). The δ18O record of the shallow-dwelling foraminifera G. sacculifer and the aragonite content are dominated by sub-Milankovitch variability. In contrast, magnetic susceptibility and the δ18O record of the deeper-dwelling foraminifera G. menardii show precession cycles. The relationship between the aragonite and the paleoproxy data suggests that the export of sediment from the adjacent Great Bahama Bank was triggered directly by atmospheric processes rather than by sea level change. We propose a climate mechanism that bears similarities with the semiannual cycle component of eastern equatorial Pacific sea surface temperatures under present-day conditions.

3D seismic geomorphology and sedimentology of the Chalk Group, southern Danish North Sea

Abstract: Classically, the North Sea Chalk is interpreted as having been deposited under quiet, homogeneous pelagic conditions with local redeposition in slumps and slides. Recent observations of highly discontinuous reflection patterns on 2D and 3D seismic reflection data from the NW European Chalk Group have led to a revision of some general ideas of chalk deposition, with the suggestion that long-lived, contour-parallel bottom currents exerted a primary influence on the development of intra-chalk channels, drifts and mounds. This study proposes an alternative explanation for the formation of selected intra-chalk seismic and stratal discontinuities, interpreting these as being caused by gravity-driven processes that developed in response to intense syndepositional tectonics. Submarine mass-transport systems identified in the study area include large-scale slumps, slides, debris flows and turbidites. The last occur in sinuous channel systems flanked by large master levees, with the channel fill exhibiting well-developed secondary banks and overbanks on the outer bends of the channel thalweg. This first documentation of channelized density-flow deposits in the North Sea Chalk has important consequences for the interpretation and prediction of redeposited chalk units, emphasizing at the same time the strength of detailed 3D seismic discontinuity detection for subsurface sedimentary-systems analysis.

Indonesian Throughflow drove Australian climate from humid Pliocene to arid Pleistocene

Late Miocene to mid-Pleistocene sedimentary proxy records reveal that northwest Australia underwent an abrupt transition from dry to humid climate conditions at 5.5 million years (Ma), likely receiving year-round rainfall, but after ~3.3 Ma, climate shifted toward an increasingly seasonal precipitation regime. The progressive constriction of the Indonesian Throughflow likely decreased continental humidity and transferred control of northwest Australian climate from the Pacific to the Indian Ocean, leading to drier conditions punctuated by monsoonal precipitation. The northwest dust pathway and fully established seasonal and orbitally controlled precipitation were in place by ~2.4 Ma, well after the intensification of Northern Hemisphere glaciation. The transition from humid to arid conditions was driven by changes in Pacific and Indian Ocean circulation and regional atmospheric moisture transport, influenced by the emerging Maritime Continent. We conclude that the Maritime Continent is the switchboard modulating teleconnections between tropical and high-latitude climate systems.

Australian shelf sediments reveal shifts in Miocene Southern Hemisphere westerlies

Sediments from Western Australia show how westerly winds made the southwest wetter during the Miocene (18 to 6 million years ago). Global climate underwent a major reorganization when the Antarctic ice sheet expanded ~14 million years ago (Ma) (1). This event affected global atmospheric circulation, including the strength and position of the westerlies and the Intertropical Convergence Zone (ITCZ), and, therefore, precipitation patterns (2–5). We present new shallow-marine sediment records from the continental shelf of Australia (International Ocean Discovery Program Sites U1459 and U1464) providing the first empirical evidence linking high-latitude cooling around Antarctica to climate change in the (sub)tropics during the Miocene. We show that Western Australia was arid during most of the Middle Miocene. Southwest Australia became wetter during the Late Miocene, creating a climate gradient with the arid interior, whereas northwest Australia remained arid throughout. Precipitation and river runoff in southwest Australia gradually increased from 12 to 8 Ma, which we relate to a northward migration or intensification of the westerlies possibly due to increased sea ice in the Southern Ocean (5). Abrupt aridification indicates that the westerlies shifted back to a position south of Australia after 8 Ma. Our midlatitude Southern Hemisphere data are consistent with the inference that expansion of sea ice around Antarctica resulted in a northward movement of the westerlies. In turn, this may have pushed tropical atmospheric circulation and the ITCZ northward, shifting the main precipitation belt over large parts of Southeast Asia (4).

Origin and deformation of intra-salt sulphate layers: an example from the Dutch Zechstein (Late Permian)

From salt mine galleries and well data it is known that thick rock salt layers can contain anhydrite and carbonate layers with thicknesses on the millimetre to tens of metre scale. The relatively thick Zechstein 3 anhydrite–carbonate layer in the northern Netherlands has been studied previously using 3-D seismic data. Observations from geophysical well logs in this study reveal the presence of thin sulphate layers on the sub-seismic scale imbedded in the Zechstein 2 (Z2) salt. Core samples, thin sections, seismic data and geochemical measurements were used to determine the mineralogy and origin of one of these Z2 sulphate layers. Bromine analyses show that they mark a freshening event in the Z2 salt, which can be correlated over large distances in the northern Netherlands. Their core-calibrated log signature indicates that the Z2 sulphate layers consist either of pure anhydrite or of anhydrite and polyhalite. The mineralogy and thickness of the sulphate layers are interpreted to vary between synsedimentary morphologic lows (thin anhydrite–polyhalite couplets) and highs (thicker anhydrite layers). Such a combination of core observations and well log analysis is a powerful tool to detect lateral trends in evaporite mineralogy and to reconstruct the environmental setting of their formation. Salt internal geometries can further be used to distinguish between different deformation mechanisms. In our study area, the distribution of sulphate layers within the Z2 salt indicates that subjacent salt dissolution was not the dominant process leading to salt-related deformation.

Impacts of hydrothermal dolomitization and thermochemical sulfate reduction on secondary porosity creation in deeply buried carbonates: A case study from the Lower Saxony Basin, northwest Germany

The role of deep-burial dissolution in the creation of porosity in carbonates has been discussed controversially in the recent past. We present a case study from the Upper Permian Zechstein 2 carbonate reservoirs of the Lower Saxony Basin in northwest Germany. These reservoirs are locally characterized by high amounts of carbon dioxide (CO2) and variable amounts of hydrogen sulfide (H2S), which are derived from thermochemical sulfate reduction (TSR) and inorganic sources. To study the contribution of these effects on porosity development, we combine petrography, stable isotope, and rare earth and yttrium (REY) analyses of fracture cements with Raman spectroscopy and δ13C analyses of fluid inclusions. It is shown that fluid migration along deep fault zones created and redistributed porosity. Fluid inclusion analyses of vein cements demonstrate that hydrothermal fluids transported inorganic CO2 into the reservoir, where it mixed with minor amounts of TSR-derived organic CO2. The likely source of inorganic CO2 is the thermal decomposition of deeply buried Devonian carbonates. The REY distribution patterns support a hydrothermal origin of ascending iron- and CO2-rich fluids causing dolomitization of calcite and increasing porosity by 10%–16% along fractures. This porosity increase results from hydrothermal dolomitization and dissolution by acids generated from the reaction of Fe2+ with H2S to precipitate pyrite. In contrast, hydrothermal dolomite cements reduced early diagenetic porosity in dolomitic intervals by approximately 17%. However, the carbonate dissolution in the predominantly calcitic host rock results in a net increase in porosity and permeability in the vicinity of the fracture walls, which has to be considered for modeling reservoir properties and fluid migration pathways.