Sea-level responses to rapid sediment erosion and deposition in Taiwan

Numerous studies have shown that sediment deposition can perturb sea level by several meters over millennial timescales by modifying the gravity field, crustal elevation, and sediment thickness. Relatively few studies have focused on the complementary role of erosion on sea-level change despite its effects on the same quantities, partly because many rapidly eroding mountains are too far from shorelines to strongly perturb sea level at the coast. Taiwan, a mountainous island eroding rapidly within tens of km of the coast, offers an opportunity to investigate the joint influences of rapid onshore erosion and rapid offshore deposition on sea-level change. Here we develop a sediment loading history for Taiwan since the previous interglacial (∼120 ka) by compiling published erosion and deposition rate measurements and by applying a geometric marine sediment deposition and compaction model for sites without deposition rate measurements. We use the resulting sediment redistribution history to drive sea-level responses in a gravitationally self-consistent sea-level model. Our simulations show that the effects of rapid onshore erosion outweigh the effects of rapid offshore deposition along Taiwan's east coast. Along the east coast of Taiwan, sediment redistribution induces rapid sea-level fall, a response that differs in sign from the coastal sea-level rise induced by rapid sediment redistribution in many other river systems around the world. The spatial extent of the modeled sea-level fall is sensitive to the Earth model, particularly the effective elastic thickness of the lithosphere, a sensitivity that we describe in further detail in the Discussion. These results suggest that sediment redistribution could have generated sea-level changes of >10m on the east coast of Taiwan since 10 ka and >100m since 120 ka. This can account for some of the discrepancy between observed and modeled paleo-sea-level marker elevations, which reduces estimates of tectonically driven rock uplift rates inferred from the elevation differences between paleo-sea-level markers and modeled sea level. This highlights the importance of accounting for erosional unloading in interpretations of paleo-sea-level reconstructions and associated estimates of tectonically driven uplift rates

Orbitally forced ice sheet fluctuations during the Marinoan Snowball Earth glaciation

Two global glaciations occurred during the Neoproterozoic. Snowball Earth theory posits that these were terminated after millions of years of frigidity when initial warming from rising atmospheric CO2 concentrations was amplified by the reduction of ice cover and hence a reduction in planetary albedo. This scenario implies that most of the geological record of ice cover was deposited in a brief period of melt-back. However, deposits in low palaeo-latitudes show evidence of glacial–interglacial cycles. Here we analyse the sedimentology and oxygen and sulphur isotopic signatures of Marinoan Snowball glaciation deposits from Svalbard, in the Norwegian High Arctic. The deposits preserve a record of oscillations in glacier extent and hydrologic conditions under uniformly high atmospheric CO2 concentrations. We use simulations from a coupled three-dimensional ice sheet and atmospheric general circulation model to show that such oscillations can be explained by orbital forcing in the late stages of a Snowball glaciation. The simulations suggest that while atmospheric CO2 concentrations were rising, but not yet at the threshold required for complete melt-back, the ice sheets would have been sensitive to orbital forcing. We conclude that a similar dynamic can potentially explain the complex successions observed at other localities

Phosphorus sources for phosphatic Cambrian carbonates

The fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating, but time-bounded, window into Ediacaran–Cambrian animal evolution. For reasons that remain unknown, phosphatic fossil preservation declined signifi cantly through Cambrian Series 2. Here, we investigate the phosphorus (P) sources for phosphatic Cambrian carbonates, presenting sedimentological, petrographic, and geochemical data from the Cambrian Series 2–3 Thorntonia Limestone, Australia, some of the youngest Cambrian strata to display exceptional phosphatic preservation of small shelly fossils. We fi nd that within Thorntonia sediments, phosphate was remobilized by organic decay and bacterial iron reduction, with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils. We discuss the merits of bioclastic-derived, organic matter–bound, or iron-bound P as potential sources to these strata. Petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the P for fossil preservation. In contrast, high organic carbon contents imply signifi cant organic fl uxes of P to Thorntonia sediments. Sedimentology and iron-speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic, ferruginous conditions in subsurface water masses, suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second signifi cant P source to Thorntonia sediments. Simple stoichiometric models suggest that, by themselves, neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment. Thus, we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the Thorntonia Limestone

Dynamic topography and ice age paleoclimate

The connection between the geological record and dynamic topography driven by mantle convective flow has been established over widely varying temporal and spatial scales. As observations of the process have increased and numerical modeling of thermochemical convection has improved, a burgeoning direction of research targeting outstanding issues in ice age paleoclimate has emerged. This review focuses on studies of the Plio-Pleistocene ice age, including investigations of the stability of ice sheets during ice age warm periods and the inception of Northern Hemisphere glaciation. However, studies that have revealed nuanced connections of dynamic topography to biodiversity, ecology, ocean chemistry, and circulation since the start of the current ice-house world are also considered. In some cases, a recognition of the importance of dynamic topography resolves enigmatic events and in others it confounds already complex, unanswered questions. All such studies highlight the role of solid Earth geophysics in paleoclimate research and undermine a common assumption, beyond the field of glacial isostatic adjustment, that the solid Earth remains a rigid, passive substrate during the evolution of the ice age climate system