Search

Climate oscillations preserved in sedimentary archives tend to decrease in resolution further back in Earth's history. High‐frequency climate cycles (e.g., ∼20‐Kyr precession cycles) are especially prone to poor preservation due to sediment reworking. Recent studies have shown, however, that given sufficient basin accommodation space and sedimentation rate, shallow‐marine paleoclimate archives record precession‐driven hydroclimate change in mid‐low latitude regions. Our study evaluates how the evolution of a rapidly uplifting orogen influences the recording of astronomical climate forcing in shallow‐marine sedimentary strata in the Taiwan Western Foreland Basin (WFB). Time‐series analysis of gamma‐ray records through the late Miocene–Pliocene Kueichulin Formation shows that during early stages of Taiwan orogenesis (before 5.4 Ma), preservation of precession‐driven East Asian Summer Monsoon variability is low despite increasing monsoon intensities between 8 and 3 Ma. The Taiwan Strait had not formed, and the southeast margin of Eurasia was open to the Pacific Ocean. Consequently, depositional environments in the WFB were susceptible to reworking by large waves, resulting in the obscuration of higher‐frequency precession cycles. From 5.4 to 4.92 Ma, during early stages of emergence of Taiwan, basin subsidence increased while sedimentation rates remained low, resulting in poor preservation of orbital oscillations. After 4.92 Ma and up to 3.15 Ma, Taiwan became a major sediment source to the WFB, and sheltered the WFB from erosive waves with the development of Taiwan Strait. The elevated sediment influx, increased basin accommodation as the WFB developed, and formation of a semi‐sheltered strait, resulted in enhanced preservation of precession‐driven East Asian Summer Monsoon variability. Rhythmic changes in the shape of Earth's orbit (eccentricity, ∼100‐Kyr cycles), tilt (obliquity, ∼41‐Kyr cycles), and axial rotation (precession, ∼20‐Kyr cycles) drive climate change that may be preserved in sedimentary records. Orbital climate cycles are recorded in clastic shallow‐marine strata where the amount of sediment transported from land to sea and space available for deposition in the basin are sufficiently high. Our study of the Kueichulin Formation in the Taiwan Western Foreland Basin (WFB) shows how a rapidly uplifting orogen influences the preservation of different climate cycles in the shallow‐marine realm. During early Taiwan orogenesis (before 5.4 Ma) when the Taiwan Strait did not exist, sediment in the WFB were susceptible to reworking by erosive waves generated in the Pacific Ocean, which obscured higher‐frequency precession cycles. From 5.4 to 4.92 Ma, Taiwan began to emerge from the Pacific Ocean and the WFB began to subside, but sedimentation rates were low, so climate cycles were poorly preserved. After 4.92 Ma, rapid uplift and erosion of Taiwan and WFB subsidence continued, resulting in increased sedimentation to the sea, increased sediment accumulation space, and the formation of a strait that sheltered the WFB from waves, which all served to enhance the preservation of precession. Preservation of different climate cycles in shallow‐marine records is influenced by a rapidly uplifting orogenDuring early uplift of Taiwan (before 5.4 Ma), only eccentricity and obliquity were preserved due to low sedimentation/basin accommodationFormation of a sheltered strait and increased sediment flux/basin subsidence as Taiwan uplifted enhanced preservation of precession cycles Preservation of different climate cycles in shallow‐marine records is influenced by a rapidly uplifting orogen During early uplift of Taiwan (before 5.4 Ma), only eccentricity and obliquity were preserved due to low sedimentation/basin accommodation Formation of a sheltered strait and increased sediment flux/basin subsidence as Taiwan uplifted enhanced preservation of precession cycles

In Chignecto Bay, upper Bay of Fundy, Canada, the muddy tidal flats exhibit distinctive sedimentological and ichnological characteristics indicative of winter conditions and the development of ice. From late spring to autumn (May–October: mean temperature +13.6 °C), the mud flats sustain a high infaunal biomass and sediment deposition is dominated by tidal processes. Neap–spring tidal rhythmites, fluid-mud deposition and high levels of bioturbation are all characteristic of summer deposits. During the winter, temperatures remain below zero (December–March: mean temperature −6.3 °C), and ice forms in the bay and periodically on the mud flats. Ice rafts and blocks are common on the tidal-flat surface, and these blocks deform the muddy sediment, cut deep scours and deposit allochthonous sediment (including gravel) across the flats. Intermittent storms in the autumn and winter also contribute to sediment scouring and erosion. Annual die-offs of infauna are reflected by reduced bioturbation in winter deposits. Each spring, renewed larval recruitment and opportunistic colonization results in increasing levels of bioturbation, and the trace suite is dominated by a low-diversity assemblage of diminutive, vertical burrows. The summer–winter cyclicity in infaunal colonization manifests, ichnologically, as a distinctive bioturbated–non-bioturbated bedset character.Infaunal populations in Chignecto Bay peak in the late spring and in the late summer–early autumn. Both population peaks are exploited by vertebrates, including migrating Atlantic sturgeon (late spring) and migratory shorebirds (late summer–early autumn). In particular, the upper Bay of Fundy, including Chignecto Bay, is a staging ground for approximately 42–74% of the world’s Semipalmated sandpipers (Calidris pusilla) on their annual migration from the Arctic to South America. Fortunately, anthropogenic modification of Chignecto Bay, and its associated bays and rivers (e.g. construction and dismantling of a causeway over the Petitcodiac River), has not had a significant effect, to date, on the mud-flat habitat or the infaunal biomass.

Computer modeling of trace fossils (Skolithos, Thalassinoides, Planolites, Zoophycos, and Phycosiphon) and ichnofacies (Skolithos, Cruziana, and Zoophycos ichnofacies) is undertaken to assess the impact of bioturbation on porosity and permeability trends in sedimentary media. Model volumes are randomly populated with the digitally modeled trace fossils to test for connectivity between burrows. The probability of vertical and lateral interconnections is compared with bioturbation intensity. The results of the simulations indicate that biogenic flow networks develop at low bioturbation intensity, between 10 and 27.5% bioturbation (BI-2). However, the efficiency of connectivity is controlled by the architecture of the burrows. For all trace-fossil and ichnofacies models, regardless of trace fossil orientation, continuous horizontal and vertical connectivity across the sediment volume is achieved within a 0 to 10% range in bioturbation. In subsurface aquifers and petroleum reservoirs, the presence of bioturbation can significantly influence fluid flow. In particular, for marine sedimentary rocks, where burrows are more permeable than the surrounding matrix, a greater degree of three-dimensional burrow connectivity can produce preferred fluid-flow pathways through the rock. Recognizing these flow conduits may enable optimization of resource exploitation or may contribute to increasing reserve estimates from previously interpreted nonreservoir rock. [ABSTRACT FROM AUTHOR]