Albert Matter

Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago.

Variations in the amount of solar radiation reaching the Earth are thought to influence climate, but the extent of this influence on timescales of millennia to decades is unclear. A number of climate records show correlations between solar cycles and climate, but the absolute changes in solar intensity over the range of decades to millennia are small and the influence of solar flux on climate is not well established. The formation of stalagmites in northern Oman has recorded past northward shifts of the intertropical convergence zone, whose northward migration stops near the southern shoreline of Arabia in the present climate. Here we present a high-resolution record of oxygen isotope variations, for the period from 9.6 to 6.1 kyr before present, in a Th–U-dated stalagmite from Oman. The δ18O record from the stalagmite, which serves as a proxy for variations in the tropical circulation and monsoon rainfall, allows us to make a direct comparison of the δ18O record with the Δ14C record from tree rings, which largely reflects changes in solar activity. The excellent correlation between the two records suggests that one of the primary controls on centennial- to decadal-scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation.

Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman

Biostratigraphic, carbon isotope, and U-Pb zircon geochronological data from the Ara Group of Oman indicate an abrupt last appearance of Cloudina and Namacalathus coincident with a large-magnitude, but short-lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the Precambrian-Cambrian boundary. U-Pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542.0 ± 0.3 Ma, consistent with previous age constraints from Siberia and Namibia. Combined with the global biostratigraphic record, these new data strengthen hypotheses invoking mass extinction within terminal Proterozoic ecosystems at or near the Precambrian-Cambrian boundary.

Timing and climatic impact of Greenland interstadials recorded in stalagmites from northern Turkey.

A 50 kyr-long exceptionally well-dated and highly resolved stalagmite oxygen (δ 18O) and carbon (δ 13C) isotope record from Sofular Cave in northwestern Turkey helps to further improve the dating of Greenland Interstadials (GI) 1, and 3–12. Timing of most GI in the Sofular record is consistent within ±10 to 300 years with the “iconic” Hulu Cave record. Larger divergences (>500 years) between Sofular and Hulu are only observed for GI 4 and 7. The Sofular record differs from the most recent NGRIP chronology by up to several centuries, whereas age offsets do not increase systematically with depth. The Sofular record also reveals a rapid and sensitive climate and ecosystem response in the eastern Mediterranean to GI, whereas a phase lag of ∼100 years between climate and full ecosystem response is evident. Finally, results of spectral analyses of the Sofular isotope records do not support a 1,470-year pacing of GI.

Speleothem evidence from Oman for continental pluvial events during interglacial periods

Growth periods and stable isotope analyses of speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods extending back over the past four of Earth’s glacial-interglacial cycles. Rapid speleothem growth occurred during the early to middle Holocene (6‐10.5 ka B.P.), 78‐82 ka B.P., 120‐135 ka B.P., 180‐200 ka B.P., and 300‐325 ka B.P. The speleothem calcite deposited during each of these episodes is highly depleted in 18 O compared to modern speleothems. The d 18 O values for calcite deposited within pluvial periods generally fall in the range of 24‰ to 28‰ relative to the Vienna Peedee belemnite standard, whereas modern speleothems range from 21‰ to 23‰. The growth and isotopic records indicate that during peak interglacial periods, the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coincident with an interglacial stage of the marine oxygen isotope record. The association of continental pluvial periods with peak interglacial conditions suggests that glacial boundary conditions, and not changes in solar radiation, are the primary control on continental wetness on glacial-interglacial time scales.

A 780-year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman

[1] Meteorological records of monsoon rainfall in the Indian Ocean are generally less than 100 years long. The relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales, to determine what factors influence the intensity of rainfall on these timescales, or to place possible changes in the twentieth century into a broader historical context. Development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward. We have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern Arabia. Our results show that monsoon variation over the past century is not outside of the range of the past 800 years. Decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the Indian Ocean. Spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales. Decadal to interannual cycles in the record appear to originate in the tropical Pacific Ocean. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 1620 Global Change: Climate dynamics (3309); 1699 Global Change: General or miscellaneous; KEYWORDS: Monsoon, paleoclimate, rainfall, Indian Ocean, speleothem, oxygen isotopes

Changing moisture sources over the last 330,000 years in Northern Oman from fluid-inclusion evidence in speleothems

Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δD values of water extracted from speleothem fluid inclusions (δDFI) are between −60 and −20‰ (VSMOW) and δ18O values of speleothem calcite (δ18OC) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δD) or modern stalagmites (for δ18O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ18O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods.

Speleothem-based paleoclimate record from northern Oman

U-Th age dating and stable isotope measurements of speleothems from Hoti Cave in northern Oman yield paleoclimate information from the region extending to 125 ka. The results (1) provide further confirmation of an early Holocene wet period in southern Arabia extending from some time prior to 9.7 ka and ending at 6.2 ka; (2) demonstrate a second period of wetness closely coinciding with the last interglacial period, marine isotope stage (MIS) 5e; (3) indicate that during MIS 5e, southern Arabia was considerably wetter than during the early Holocene; and (4) demonstrate that periods of increased monsoon wind strength, based on data from marine sediments, do not always coincide with evidence of greatly increased precipitation even from nearby continental areas.

Timing and structure of the 8.2 kyr B.P. event inferred from δ18O records of stalagmites from China, Oman, and Brazil

Oxygen isotope records of stalagmites from China and Oman reveal a weak summer monsoon event, with a double-plunging structure, that started 8.21 ± 0.02 kyr B.P. An identical but antiphased pattern is also evident in two stalagmite records from eastern Brazil, indicating that the South American Summer Monsoon was intensifi ed during the 8.2 kyr B.P. event. These records demonstrate that the event was of global extent and synchronous within dating errors of <50 years. In comparison with recent model simulations, it is plausible that the 8.2 kyr B.P. event can be tied in changes of the Atlantic Meridional Overturning Circulation triggered by a glacial lake draining event. This, in turn, affected North Atlantic climate and latitudinal position of the Intertropical Convergence Zone, resulting in the observed low-latitude monsoonal precipitation patterns.

Diagenesis and Fluid Evolution of Deeply Buried Permian (Rotliegende) Gas Reservoirs, Northwest Germany

Depositional environment and tectonic setting were important in the diagenesis and evolution of reservoir properties in the Rotliegende sequence of the North German Basin. Facies belts paralleling the edge of a central saline lake controlled the distribution of early and shallow burial cements. Lake shoreline sands with radial chlorite cement show the best reservoir properties in the study area. Juxtaposition of Rotliegende deposits against either Carboniferous Coal Measures or Late Permian (Zechstein) evaporites by faulting resulted in cross-formational fluid exchange. The introduction of fluids from Carboniferous Coal Measures into Rotliegende reservoirs produced intense clay cementation, significantly reducing rock permeabilities. Influx of Zechstein fluids favored pre ipitation of late carbonate and anhydrite cements. Cross-formational and fault-related fluid flow was enhanced during periods of fault activity.

Timing diagenesis in the Tartan Reservoir (UK North Sea): constraints from combined cathodoluminescence microscopy and fluid inclusion studies

Abstract Fluid inclusions occur in authigenic quartz, baryte, ankerite and calcite cements that partially occlude inferred secondary porosity in Piper Formation sandstones of the Tartan field and adjacent Witch Ground Graben. Detailed petrography combined with hot cathodoluminescence studies enables a fluid inclusion stratigraphy to be related to the authigenic minerals and individual cement zones within them. Microthermometric studies indicate that these inclusions formed at elevated temperatures in the range 70–120°C. These temperatures are interpreted to indicate hot migrating fluids which invaded the on-structure sandstones at burial depths exceeding 1.5 km during the late Cretaceous-early Tertiary. Between successive cement generations major fluctuations in salinity are recorded. There is a marked lowering of salinity between subsequent quartz generations. Baryte and calcite cements, associated with metal sulphides, were precipitated from a low salinity brine. Ankerite cement at the present oil-water contact records precipitation from high salinity pore fluids. The salinity variations are thought to reflect large scale sub-surface fluid migration. Sources of acidity for the generation of secondary porosity and sources of silica, sulphate and base metals for the pervasive late cements cannot all be found within the reservoir sequence. This therefore indicates mass transfer within an open system. The complex diagenetic assemblage is unlikely to have been precipitated from the evolution of a single pore fluid. Mixing of at least two sub-surface fluids of very different chemistry and origin is inferred. Sulphate for barytes and sulphide cements is thought to have been derived as a result of cross-formational flow from Zechstein anhydrite juxtaposed against the reservoir prior to oil migration. Homogenization temperatures of fluid inclusions within cements along major fault planes are offset towards significantly higher temperatures than those inclusions distanced from such faults. This, together with the distribution of porosity and cements on the crestal structures and adjacent to major faults, strongly suggests that the faults were the condults for migrating hot fluids which mixed with the sulphate brine on the Tartan structure. Seismic valving provides an elegant mechanism for transporting large volumes of hot fluids intermittently but repeatedly along the major bounding faults. The source of the hot fluid is inferred to be related to the dehydration and illitization of smectite reaction, maturation of organic matter and expulsion of pore waters in the adjacent Witch Ground Graben, all of which took place broadly coincident with the generation of secondary porosity and late on-structure cementation.