Jens Zinke

On the role of the Agulhas system in ocean circulation and climate

The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.

Tropical sea surface temperatures for the past four centuries reconstructed from coral archives

Most annually resolved climate reconstructions of the Common Era are based on terrestrial data, making it a challenge to independently assess how recent climate changes have affected the oceans. Here as part of the Past Global Changes Ocean2K project, we present four regionally calibrated and validated reconstructions of sea surface temperatures in the tropics, based on 57 published and publicly archived marine paleoclimate data sets derived exclusively from tropical coral archives. Validation exercises suggest that our reconstructions are interpretable for much of the past 400 years, depending on the availability of paleoclimate data within, and the reconstruction validation statistics for, each target region. Analysis of the trends in the data suggests that the Indian, western Pacific, and western Atlantic Ocean regions were cooling until modern warming began around the 1830s. The early 1800s were an exceptionally cool period in the Indo-Pacific region, likely due to multiple large tropical volcanic eruptions occurring in the early nineteenth century. Decadal-scale variability is a quasi-persistent feature of all basins. Twentieth century warming associated with greenhouse gas emissions is apparent in the Indian, West Pacific, and western Atlantic Oceans, but we find no evidence that either natural or anthropogenic forcings have altered El Nino–Southern Oscillation-related variance in tropical sea surface temperatures. Our marine-based regional paleoclimate reconstructions serve as benchmarks against which terrestrial reconstructions as well as climate model simulations can be compared and as a basis for studying the processes by which the tropical oceans mediate climate variability and change.

Mayotte coral reveals hydrological changes in the western Indian Ocean between 1881 and 1994

We reconstruct the hydrologic history of the tropical western Indian Ocean by calculating the δ18Oseawater from coupled coral Sr/Ca and δ18O measurements in a massive Porites coral from Mayotte (Comoros) between 1881 and 1994. We found that the precipitation-evaporation balance varies naturally on time scales of 5–6 years and 18–25 years. High (low) SSTs are associated with positive (negative) δ18Oseawater implying that atmospheric variability is linked with remote climate modes in the Indian Ocean and the tropical/extratropical Pacific Ocean. Warm El Nino-Southern Oscillation events are associated with a negative freshwater balance at Mayotte. This case study demonstrates that a much denser network of δ18Oseawater reconstructions is crucial for understanding the spatial patterns of hydrological conditions.

Postglacial flooding history of Mayotte Lagoon (Comoro Archipelago, southwest Indian Ocean)

Abstract Four cores from the fringing reefs and five sediment cores from Mayotte Lagoon, Comoro Archipelago, southwest Indian Ocean, which reached the Pleistocene/Holocene boundary, form the database of this study. They offer the opportunity to reexamine and complete the postglacial sea-level curve, especially for the time interval between 11.6 to 8 kyr cal BP. Between 11.6 kyr cal BP until present the history of sea-level rise showed the following steps: (1) by about 19 mm/yr between 11.6 and 9.6 kyr cal BP, (2) by 9 mm/yr between 9.6 and 8 kyr cal BP, (3) by 3 mm/yr between 8 and 7 kyr cal BP, and (4) by 0.9 mm/yr after 7 kyr cal BP until stabilisation at present level at 2.5 kyr cal BP. In addition, a decline in the rates of sea-level rise or even a stillstand is observed between 13 to 11.6 kyr cal BP. The flooding of the lagoon of Mayotte was controlled by the depth of the reefal passages, which were cut by rivers and/or due to erosion during the time of emergence since the last interglacial. The differences in the shape of the sea-level curve from Mayotte compared to those from other sites located far from the former glaciated regions are related to: (1) the small size of the island, (2) the rapid downward movement of this small volcanic island with the oceanic plate into the mantle due to hydro–isostatic compensation after addition of meltwater, and (3) the location between large continents.

Small theropod teeth from the Upper Jurassic coal mine of Guimarota (Portugal)

KurzfassungIsolierte Zähne theropoder Dinosaurier aus dem Oberen Jura der Kohlengrube Guimarota (bei Leiria, Portugal) werden beschrieben und abgebildet. Die häufigsten Zahnfunde ähneln morphologisch den Bezahnungen der oberjurassischen TheropodenArchaeopteryx undCompsognathus. Ein Morphotyp deutet auf das Vorhandensein eines Allosauriden hin. Sechs weitere Morphotypen von Theropoden-Zähnen zeigen große Ähnlichkeiten mit kreidezeitlichen Formen, wie Dromaeosauriden, Troodontiden, Tyrannosauriden,Richardoestesia undParonychodon. Für diese Gruppen wird ein spätjurassischer Ursprung diskutiert, wie er von einigen Autoren bereits vermutet wurde.AbstractIsolated teeth of small theropod dinosaurs from the Upper Jurassic lignite coal mine of Guimarota (near Leiria, Portugal) are described and illustrated. The well known Upper Jurassic theropods from Europe,Archaeopteryx andCompsognathus, are the most common taxa in the Guimarota assemblage. One morphotype is closely related to an allosaurid theropod. Six further morphotypes of theropod teeth are also described, which are closely related to Cretaceous theropods such as dromaeosaurids, troodontids, tyrannosaurids,Richardoestesia andParonychodon. A Late Jurassic origin of these groups of theropods, which is very often postulated, is discussed.

Corals record long-term Leeuwin current variability including Ningaloo Niño/Niña since 1795

Variability of the Leeuwin current (LC) off Western Australia is a footprint of interannual and decadal climate variations in the tropical Indo-Pacific. La Niña events often result in a strengthened LC, high coastal sea levels and unusually warm sea surface temperatures (SSTs), termed Ningaloo Niño. The rarity of such extreme events and the response of the southeastern Indian Ocean to regional and remote climate forcing are poorly understood owing to the lack of long-term records. Here we use well-replicated coral SST records from within the path of the LC, together with a reconstruction of the El Niño-Southern Oscillation to hindcast historical SST and LC strength from 1795 to 2010. We show that interannual and decadal variations in SST and LC strength characterized the past 215 years and that the most extreme sea level and SST anomalies occurred post 1980. These recent events were unprecedented in severity and are likely aided by accelerated global ocean warming and sea-level rise.

Caribbean coral tracks Atlantic Multidecadal Oscillation and past hurricane activity

It is highly debated whether global warming contributed to the strong hurricane activity observed during the last decade. The crux of the recent debate is the limited length of the reliable instrumental record that exacerbates the detection of possible long-term changes in hurricane activity, which naturally exhibits strong multidecadal variations that are associated with the Atlantic Multidecadal Oscillation (AMO). The AMO, itself a major mode of climate variability, remains also poorly understood because of limited data. Here, we present the first coral-based proxy record (δ18O) that clearly captures multidecadal variations in the AMO and the hurricane activity. Our record, obtained from a brain coral situated in the Atlantic hurricane domain, is equally sensitive to variations in sea surface temperature (SST) and seawater δ18O, with the latter being strongly linked to precipitation, by this means amplifying large-scale climate signals in coral δ18O. The SST and precipitation signals in the coral provide the longest, thus far, continuous proxy-based record of hurricane activity that interestingly exhibits a long-term increase over the last century. As multidecadal SST variations in this region are closely related to the AMO, this study raises new possibilities to extend the limited observations and to gain new insights into the mechanisms underlying the AMO and long-term hurricane variations.

Human deforestation outweighs future climate change impacts of sedimentation on coral reefs.

Near-shore coral reef systems are experiencing increased sediment supply due to conversion of forests to other land uses. Counteracting increased sediment loads requires an understanding of the relationship between forest cover and sediment supply, and how this relationship might change in the future. Here we study this relationship by simulating river flow and sediment supply in four watersheds that are adjacent to Madagascar’s major coral reef ecosystems for a range of future climate change projections and land-use change scenarios. We show that by 2090, all four watersheds are predicted to experience temperature increases and/or precipitation declines that, when combined, result in decreases in river flow and sediment load. However, these climate change-driven declines are outweighed by the impact of deforestation. Consequently, our analyses suggest that regional land-use management is more important than mediating climate change for influencing sedimentation of Malagasy coral reefs.

Seismic architecture and sediment distribution within the Holocene barrier reef-lagoon complex of Mayotte (Comoro archipelago, SW Indian Ocean)

Twenty gravity cores and a large set of high-resolution seismic profiles from various lagoonal settings were studied to determine the Holocene sediment distribution and sequence architecture within the Mayotte barrier reef–lagoon complex. The Holocene seismic sequence comprises a type 1 sequence with lowstand, transgressive and highstand systems tracts. The lowstand systems tract consists of a paleosoil horizon formed during subaerial exposure. The transgressive systems tract is composed of four depositional systems: (1) inner transgressive layer, (2) proximal and distal incised valley fills, (3) mid-lagoonal layer and (4) keep-up or catch-up fringing and barrier reef sequence. The highstand systems tract comprises three depositional systems: (1) of a proximal terrigenous wedge, (2) mid-lagoonal and distal carbonate sands or muds and (3) reefal carbonates. Our studies show that the nature of the Holocene sequence is controlled by the rate and amplitude of sea-level rise and environmental changes, which are expressed by changes in clastic sediment supply and carbonate production. The pre-Holocene topography and water dynamics steer the vertical and spatial sediment thickness distribution of the Holocene. Additional important parameters are the proximity to a source area (carbonate or terrigenous) and the width of the depositional area. Climate dynamics are also of great importance while they determine carbonate production and terrigenous runoff. Sedimentation rates in the subtidal settings always lacked behind sea-level rates. Thus, a steep relief was created keeping most lagoonal parts within the deep subtidal realm in which sediment production was not efficient enough to fill up accommodation space. In addition, wave and/or current energy might prevent the fill up of the lagoon. This ultimately resulted in a typical empty bucket morphology. Only a high amplitude sea-level fall would allow the subtidal lagoon to build up to base-level. Unfilled accommodation space, therefore, must be a very common feature in the geologic record.

Multi-proxy summer and winter precipitation reconstruction for southern Africa over the last 200 years

This study presents the first consolidation of palaeoclimate proxy records from multiple archives to develop statistical rainfall reconstructions for southern Africa covering the last two centuries. State-of-the-art ensemble reconstructions reveal multi-decadal rainfall variability in the summer and winter rainfall zones. A decrease in precipitation amount over time is identified in the summer rainfall zone. No significant change in precipitation amount occurred in the winter rainfall zone, but rainfall variability has increased over time. Generally synchronous rainfall fluctuations between the two zones are identified on decadal scales, with common wet (dry) periods reconstructed around 1890 (1930). A strong relationship between seasonal rainfall and sea surface temperatures (SSTs) in the surrounding oceans is confirmed. Coherence among decadal-scale fluctuations of southern African rainfall, regional SST, SSTs in the Pacific Ocean and rainfall in south-eastern Australia suggest SST-rainfall teleconnections across the southern hemisphere. Temporal breakdowns of the SST-rainfall relationship in the southern African regions and the connection between the two rainfall zones are observed, for example during the 1950s. Our results confirm the complex interplay between large-scale teleconnections, regional SSTs and local effects in modulating multi-decadal southern African rainfall variability over long timescales.