Late Oligocene rapid transformations in the South China Sea

Lithobiostratigraphic data indicate that the double reflectors on the seismic profile through Ocean Drilling Program (ODP) Site 1148 represent two unconformities that coincide, respectively, with the lower/upper Oligocene boundary at ~488 mcd, and Oligocene-Miocene boundary at 460 mcd. Two other unconformities, at ~478 and 472 mcd, respectively, were also identified within the upper Oligocene section. Together they erased a sediment record of about 3 Ma from this locality in a period of very active seafloor spreading. The existence of 32.8 Ma marine sediment at the terminated depth (850 mcd) indicates that the initial breakup of the South China Sea (SCS) was probably during 34-33 Ma, close to the Eocene-Oligocene boundary. High sedimentation rates of 60-115 m/my from the much expanded, N350 m lower Oligocene section resulted from rifting and rapid subsidence between 33 and 29 Ma. The mid-Oligocene unconformity at ~28.5 Ma, which also occurred in many parts of the Indo-West Pacific region, was probably related to a significant uplift of the Himalayan-Tibetan Plateau to the west and the initial collision between Indonesia and Australia in the south. A narrowed Indonesian seaway may have accounted for the late Oligocene warming and chalk deposition in the northern South China Sea including the Site 1148 locality. The unconformities and slumps near the Oligocene-Miocene boundary indicate a very unstable tectonic regime, probably corresponding to changes in the rotation of different land blocks and the seafloor spreading ridge from nearly E-W to NE-SW, as recognized earlier at magnetic Anomaly 7. This 25 Ma event also saw the first New Guinea terrane docking at the northern Australian craton. The low sedimentation rate of ~15 m/my in the early to middle Miocene may correspond to another period of rapid seafloor spreading and rapid widespread subsidence that effectively caused sediment source areas to retreat with a rapidly rising sea level. The isostatic nature of these late Oligocene unconformities and slumps with several major collision-uplift events indicate that the rapid changes in the early evolutionary history of the South China Sea were mainly responding to regional tectonic reconfiguration including the uplift-driven southeast extrusion of the Indochina subcontinent.

Brachiopods in Early Carboniferous

Changes in Mississippian global paleogeography derived from the reconfiguration of the continents, a reversal in ocean currents and global cooling. Although the tectonic and climatic changes are well-documented, their effects on the distribution of brachiopod fauna are poorly documented. Here we present systematic quantitative analyses on global paleobiogeography based on a global brachiopod database from the Mississippian (i.e., Tournaisian, Visean, and Serpukhovian). The dataset consists of 2123 species of 344 brachiopod genera from 1156 localities. Our results reveal that global provincialism was not evident during the Tournaisian and Visean Stages. Two realms, i.e., the Gondwanan and Paleoequatorial Realms, are recognized during the Tournaisian. The Paleoequatorial Realm dominates during the Visean Stage, whereas the Gondwanan Realm is not documented due to the absence of data points. In contrast to the early and middle Mississippian stages, faunal provincialism is greatly enhanced in the Serpukhovian Stage with Paleotethyan and North American realms easily distinguished. This indicates that the Rheic Ocean was closed before the Serpukhovian due to the collision between Gondwana and Laurussia, that disrupted faunal interchange between the Paleotethys and North America. In addition, the paleolatitude-related thermal gradient was enhanced and the Boreal Realm was distinguished from the Paleotethyan Realm during the onset of the Late Palaeozoic Ice Age (LPIA) in the Serpukhovian. The paleolatitude diversity gradient pattern further shows a distinct shift of diversity center from the southern tropic zone in the Tournaisian and Visean to the northern tropic zone in the Serpukhovian.

Late Miocene and Pliocene dinoflagellate cysts, acritarchs and terrestrial palynomorphs from ODP Hole 642B, Vøring Plateau, Norwegian Sea

The globally warm climate of the early Pliocene gradually cooled from 4 million years ago, synchronous with decreasing atmospheric CO2 concentrations. In contrast, palaeoceanographic records indicate that the Nordic Seas cooled during the earliest Pliocene, before global cooling. However, a lack of knowledge regarding the precise timing of Nordic Seas cooling has limited our understanding of the governing mechanisms. Here, using marine palynology, we show that cooling in the Nordic Seas was coincident with the first trans-Arctic migration of cool-water Pacific mollusks around 4.5 million years ago, and followed by the development of a modern-like Nordic Seas surface circulation. Nordic Seas cooling precedes global cooling by 500,000 years; as such, we propose that reconfiguration of the Bering Strait and Central American Seaway triggered the development of a modern circulation in the Nordic Seas, which is essential for North Atlantic Deep Water formation and a precursor for more widespread Greenland glaciation in the late Pliocene.

Glaisdale Beck diversion scheme - channel change and suspended sediment response

This paper describes the implementation of a novel mitigation approach and subsequent adaptive management, designed to reduce the transfer of fine sediment in Glaisdale Beck; a small upland catchment in the UK. Hydro-meteorological and suspended sediment datasets are collected over a two year period spanning pre- and post-diversion periods in order to assess the impact of the channel reconfiguration scheme on the fluvial suspended sediment dynamics. Analysis of the river response demonstrates that the fluvial sediment system has become more restrictive with reduced fine sediment transfer. This is characterised by reductions in flow-weighted mean suspended sediment concentrations from 77.93 mg/l prior to mitigation, to 74.36 mg/l following the diversion. A Mann-Whitney U test found statistically significant differences (p < 0.001) between the pre- and post-monitoring median SSCs. Whilst application of one-way analysis of covariance (ANCOVA) on the coefficients of sediment rating curves developed before and after the diversion found statistically significant differences (p < 0.001), with both Log a and b coefficients becoming smaller following the diversion. Non-parametric analysis indicates a reduction in residuals through time (p < 0.001), with the developed LOWESS model over-predicting sediment concentrations as the channel stabilises. However, the channel is continuing to adjust to the reconfigured morphology, with evidence of a headward propagating knickpoint which has migrated 120 m at an exponentially decreasing rate over the last 7 years since diversion. The study demonstrates that channel reconfiguration can be effective in mitigating fine sediment flux in upland streams but the full value of this may take many years to achieve whilst the fluvial system, slowly readjusts.