An assessment of the influence of orbital forcing on Late Pliocene global sea-level using a shallow-marine sedimentary record from the Wanganui Basin, New Zealand

<p>Classical Milankovitch Theory suggests variance in the orbital cycles of precession (21,000 year) modulated by eccentricity (~100,000 year) and obliquity (41,000 year) should have a profound influence on polar insolation and ice volume. However, the globally-integrated ice volume proxy record (benthic δ¹⁸O) during the Late Pliocene (3.0-2.6 Ma) is dominated by obliquity-paced cycles, and lacks a significant precession component. A number of conceptual hypotheses have been proposed to explain this “41,000 year problem”, but palaeoclimate records independent of the benthic δ¹⁸O record are required to test these hypotheses. The Wanganui Basin, New Zealand, contains a well-dated, shallow-marine Neogene sedimentary succession that is widely recognised as an important site for examining sea-level/ice volume changes at orbital frequencies. In this study, the shallow-marine Late Pliocene Mangaweka Mudstone is examined at an orbital-scale resolution (~3-5 kyr sampling) along a continuous 672 metre thick (true thickness) outcropping road section on Watershed Road between the Rangitikei and Turakina River valleys. Two modern analogue-calibrated water depth proxies were used to evaluate palaeobathymetric changes: (i) sediment texture and (ii) benthic foraminifera census data. An overall trend of shallowing to inner-shelf water depths occurs up-section, but is superimposed by higher frequency fluctuations. For the lowermost ~400 metres of the section, in situ benthic foraminifera assemblages indicate water depths >100 metres. As wave-induced sand transport does not occur on the modern Manawatu-Wanganui outer-shelf, and modern wave climates are assumed to be analogous to the Pliocene, it is concluded that the sediment grainsize approach is not an appropriate proxy for reconstruction water depth changes in the lower ~400 metres of section. An integrated magneto-, bio- and tephrostratigraphy was developed that constrains the outcrop succession to between ~3.0 Ma and 2.58 Ma. Nine distinct cycles spanning ~400,000 years are identified in the grainsize and benthic foraminifera assemblages. Within the uncertainty of the age model, the Mangaweka Mudstone grainsize cycles can be matched one-for-one to the δ¹⁸O glacial-interglacial cycles, as they display a similar pattern in terms of frequency and amplitude. The frequency of the Mangaweka Mudstone cycles (and the corresponding interval in the benthic δ¹⁸O record) are dominated by the ~40,000 year obliquity cycle, but with a subordinate eccentricity component. Therefore, the fluctuations in the grainsize and benthic foraminifera proxies likely represent an indirect response to global sea-level fluctuations via their effect on continental shelf sediment transport mechanisms (non-wave) with the orbitally-paced transgression and regression of the shoreline on a restricted palaeo- continental shelf. The implications for the orbital theory of the ice ages are that during the Late Pliocene, global ice volume changes responded primarily to obliquity, and the precession influences were either: (i) too low in amplitude to have influenced the grainsize and benthic foraminifera assemblages in the Mangaweka Mudstone depositional environment, or (ii) cancelled-out in global ice volume and sea-level changes because precession forcing is anti-phased between the hemispheres.</p>

An assessment of the influence of orbital forcing on Late Pliocene global sea-level using a shallow-marine sedimentary record from the Wanganui Basin, New Zealand

<p>Classical Milankovitch Theory suggests variance in the orbital cycles of precession (21,000 year) modulated by eccentricity (~100,000 year) and obliquity (41,000 year) should have a profound influence on polar insolation and ice volume. However, the globally-integrated ice volume proxy record (benthic δ¹⁸O) during the Late Pliocene (3.0-2.6 Ma) is dominated by obliquity-paced cycles, and lacks a significant precession component. A number of conceptual hypotheses have been proposed to explain this “41,000 year problem”, but palaeoclimate records independent of the benthic δ¹⁸O record are required to test these hypotheses. The Wanganui Basin, New Zealand, contains a well-dated, shallow-marine Neogene sedimentary succession that is widely recognised as an important site for examining sea-level/ice volume changes at orbital frequencies. In this study, the shallow-marine Late Pliocene Mangaweka Mudstone is examined at an orbital-scale resolution (~3-5 kyr sampling) along a continuous 672 metre thick (true thickness) outcropping road section on Watershed Road between the Rangitikei and Turakina River valleys. Two modern analogue-calibrated water depth proxies were used to evaluate palaeobathymetric changes: (i) sediment texture and (ii) benthic foraminifera census data. An overall trend of shallowing to inner-shelf water depths occurs up-section, but is superimposed by higher frequency fluctuations. For the lowermost ~400 metres of the section, in situ benthic foraminifera assemblages indicate water depths >100 metres. As wave-induced sand transport does not occur on the modern Manawatu-Wanganui outer-shelf, and modern wave climates are assumed to be analogous to the Pliocene, it is concluded that the sediment grainsize approach is not an appropriate proxy for reconstruction water depth changes in the lower ~400 metres of section. An integrated magneto-, bio- and tephrostratigraphy was developed that constrains the outcrop succession to between ~3.0 Ma and 2.58 Ma. Nine distinct cycles spanning ~400,000 years are identified in the grainsize and benthic foraminifera assemblages. Within the uncertainty of the age model, the Mangaweka Mudstone grainsize cycles can be matched one-for-one to the δ¹⁸O glacial-interglacial cycles, as they display a similar pattern in terms of frequency and amplitude. The frequency of the Mangaweka Mudstone cycles (and the corresponding interval in the benthic δ¹⁸O record) are dominated by the ~40,000 year obliquity cycle, but with a subordinate eccentricity component. Therefore, the fluctuations in the grainsize and benthic foraminifera proxies likely represent an indirect response to global sea-level fluctuations via their effect on continental shelf sediment transport mechanisms (non-wave) with the orbitally-paced transgression and regression of the shoreline on a restricted palaeo- continental shelf. The implications for the orbital theory of the ice ages are that during the Late Pliocene, global ice volume changes responded primarily to obliquity, and the precession influences were either: (i) too low in amplitude to have influenced the grainsize and benthic foraminifera assemblages in the Mangaweka Mudstone depositional environment, or (ii) cancelled-out in global ice volume and sea-level changes because precession forcing is anti-phased between the hemispheres.</p>

Southern Ocean bottom-water cooling and ice sheet expansion during the middle Miocene climate transition

The middle Miocene climate transition (MMCT), around 14 Ma, was associated with a significant climatic shift, but the mechanisms triggering the event remain enigmatic. We present a clumped isotope (Δ47) bottom-water temperature (BWT) record from 16.0 to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean and compare it to existing BWT records from different latitudes. We show that BWTs in the Southern Ocean reached 8–10 ∘C during the Miocene climatic optimum. These high BWT values indicate considerably warmer bottom-water conditions than today. Nonetheless, bottom-water δ18O (calculated from foraminiferal δ18O and Δ47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset further demonstrates that BWTs at Site 747 were variable with an overall cooling trend across the MMCT. Notably, a cooling of around 3–5 ∘C preceded the stepped main increase in benthic δ18O, interpreted as global ice volume expansion, and appears to have been followed by a transient bottom-water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom-water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Feedbacks required for substantial ice growth and/or tectonic processes may have contributed to the observed decoupling of global ice volume and Southern Ocean BWT.

Insolation-paced sea level and sediment flux during the early Pleistocene in Southeast Asia

Global marine archives from the early Pleistocene indicate that glacial-interglacial cycles, and their corresponding sea-level cycles, have predominantly a periodicity of ~ 41 kyrs driven by Earth’s obliquity. Here, we present a clastic shallow-marine record from the early Pleistocene in Southeast Asia (Cholan Formation, Taiwan). The studied strata comprise stacked cyclic successions deposited in offshore to nearshore environments in the paleo-Taiwan Strait. The stratigraphy was compared to both a δ18O isotope record of benthic foraminifera and orbital parameters driving insolation at the time of deposition. Analyses indicate a strong correlation between depositional cycles and Northern Hemisphere summer insolation, which is precession-dominated with an obliquity component. Our results represent geological evidence of precession-dominated sea-level fluctuations during the early Pleistocene, independent of a global ice-volume proxy. Preservation of this signal is possible due to the high-accommodation creation and high-sedimentation rate in the basin enhancing the completeness of the stratigraphic record.

Reassessing global ice volume: uncertainty and structure in sea level records

&lt;p&gt;Geologically recorded sea-level variations represent the sum total of all contributing processes, be it known or unknown, and may thus help in finding the full range of future sea-level rise. Significant sea-level-rise contributions from both northern and southern ice sheets are not unprecedented in the geological record and offer a well-constrained range of natural scenarios from intervals during which ice volumes were similar to or smaller than present (i.e., interglacial periods), to intervals during which total ice volume was greater (i.e., glacial periods).&lt;/p&gt;&lt;p&gt;The last deglaciation is the most recent period of widespread destabilisation and collapse of major continental ice sheets. Records spanning the last deglaciation (as well as the ice volume maxima) are few, fragmentary and seemingly inconsistent (e.g., the timing and magnitude of melt-water pulses), in part due to locational (tectonic and glacio-isostatic) as well as modern analogue considerations (e.g., palaeo-water depth or facies formation depth). We present a new synthesis of sea-level indicators, with particular emphasis on the geological and biological context, as well as the uncertainties of each record. Using this new compilation and the novel application of statistical methods (trans-dimensional change-point analysis, which avoids &amp;#8220;overfitting&amp;#8221; of noise in the data), we will assess global ice-volume changes, sea-level fluctuations and changes in climate during the last deglaciation. Finally, we discuss the implications of these uncertainties on our ability to constrain past cryosphere changes.&lt;/p&gt;

Evolution of the climate in the next million years: A reduced-complexity model for glacial cycles and impact of fossil fuel CO2

&lt;p&gt;We propose a reduced-complexity process-based model for the long-term evolution of the global ice volume, atmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentration and global mean temperature. The model only external forcings are the orbital forcing and anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; cumulative emissions. The model consists of a system of three coupled non-linear differential equations, representing physical mechanisms relevant for the evolution of the climate &amp;#8211; ice sheets &amp;#8211; Carbon cycle system in timescales longer than thousands of years. The model is successful in reproducing the glacial-interglacial fluctuations of the last 800 kyr, in good agreement with paleorecords both in terms of timing and amplitude, with a correlation between modelled and paleo global ice volume of up to 0.86.&lt;/p&gt;&lt;p&gt;Using different model realisations, we generate a probabilistic forecast of the evolution of the Earth system over the next 1 million years under natural and several fossil-fuel CO&lt;sub&gt;2&lt;/sub&gt; release scenarios. In the natural scenario, the model assigns high probability of occurrence of long interglacials in the periods between present and 50 kyr after present, and between 400 kyr and 500 kyr after present. The next full glacial conditions are most likely to occur 90 kyr after present. The model shows that even already achieved cumulative CO&lt;sub&gt;2&lt;/sub&gt; anthropogenic emissions (500 PgC) are capable of affecting the climate evolution for up to half million years, indicating that the beginning of the next glaciation is highly unlikely in the next 150 kyr. If cumulative fossil-fuel CO&lt;sub&gt;2&lt;/sub&gt; emissions reach 3000 PgC, or higher, the model predicts with high probability ice-free Northern Hemisphere landmass conditions will prevail in the next half million years, postponing the natural occurrence of the next glacial inception to 600 kyr after present.&lt;/p&gt;

Diverse manifestations of insolation forcing of environmental changes during the middle Piacenzian

&lt;p&gt;The middle Piacenzian period is the closest sustained warm interval and a possible analog to the future climate. It is well known that global ice volume exhibits dominant 41-kyr cyclicities. However, high resolution terrestrial paleoenvironmental records are scare. Here we present a 3.6 kyr terrestrial environmental variation record from Teruel Basin of Spain and compare the results with the East Asian monsoon records. The Spain results show dominant 41-kyr cycles during the early Piacenzian (3.3-3.15 Ma) when eccentricity was at minimum, but the 41-kyr cycles weakens during the late Piacenzian 3.15-2.95 Ma when eccentricity got increased, suggesting direct forcing by insolation. This pattern is different from the monsoonal records from China, which demonstrates persistent 20-kyr cycles during the entire middle Piacenzian. The strong 41-kyr cycles in westerly region during the early Piacenzian may originate from its higher latitude and higher sensitivity to insolation gradient forcing.&lt;/p&gt;

Evolution of the climate in the next million years: A reduced-complexity model for glacial cycles and impact of anthropogenic CO₂ emissions

We propose a reduced-complexity process-based model for the long-term evolution of the global ice volume, atmospheric CO2 concentration and global mean temperature. The model only external forcings are the orbital forcing and anthropogenic CO2 cumulative emissions. The model consists of a system of three coupled non-linear differential equations, representing physical mechanisms relevant for the evolution of the Climate – Ice Sheets – Carbon cycle System in timescales longer than thousands of years. The model is successful in reproducing the glacial-interglacial cycles of the last 800 kyr, in good agreement with the timing and amplitude of paleorecord fluctuations, with the best correlation between modelled and paleo global ice volume of 0.86. Using different model realisations, we produce a probabilistic forecast of the evolution of the Earth system over the next 1 million years under natural and several fossil-fuel CO2 release scenarios. In the natural scenario, the model assigns high probability of occurrence of long interglacials in the periods between present and 120 kyr after present, and between 400 kyr and 500 kyr after present. The next glacial inception is most likely to occur ~ 50 kyr after present with full glacial conditions developing ~ 90 kyr after present. The model shows that even already achieved cumulative CO2 anthropogenic emissions (500 PgC) are capable of affecting climate evolution for up to half million years, indicating that the beginning of the next glaciation is highly unlikely in the next 120 kyr. High cumulative anthropogenic CO2 emissions (3000 PgC or higher), which could potentially be achieved in the next two to three centuries if humanity does not curb the usage of fossil-fuels, will most likely provoke Northern Hemisphere landmass ice-free conditions throughout the next half million years, postponing the natural occurrence of the next glacial inception to 600 kyr after present or later.

Geochronology and Geochemical Properties of Mid-Pleistocene Sediments on the Caiwei Guyot in the Northwest Pacific Imply a Surface-to-Deep Linkage

Seamounts are ubiquitous topographic units in the global ocean, and their effects on local circulation have attracted great research attention in physical oceanography; however, fewer relevant efforts were made on geological timescales in previous studies. The Caiwei (Pako) Guyot in the Magellan Seamounts of the western Pacific is a typical seamount and oceanographic characteristics have been well documented. In this study, we investigate a sediment core by geochronological and geochemical studies to reveal a topography-induce surface-to-bottom linkage. The principal results are as follows: (1) Two magnetozones are recognized in core MABC–11, which can be correlated to the Brunhes and Matuyama chrons; (2) Elements Ca, Si, Cl, K, Mn, Ti, and Fe are seven elements with high intensities by geochemical scanning; (3) Ca intensity can be tuned to global ice volume to refine the age model on glacial-interglacial timescales; (4) The averaged sediment accumulation rate is ~0.73 mm/kyr, agreeing with the estimate of the excess 230Th data in the upper part. Based on these results, a proxy of element Mn is derived, whose variability can be correlated with changes in global ice volume and deep-water masses on glacial-interglacial timescales. This record is also characterized by an evident 23-kyr cycle, highlighting a direct influence of solar insolation on deep-sea sedimentary processes. Overall, sedimentary archives of the Caiwei Guyot not only record an intensified abyssal ventilation during interglaciations in the western Pacific, but also provide a unique window for investigating the topography-induced linkage between the upper and bottom ocean on orbital timescales.

Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition

The middle Miocene climate transition (MMCT, ~14.5–13.0 Ma) was associated with a significant expansion of Antarctic ice, but the mechanisms triggering the event remain enigmatic. We present a new clumped isotope (∆47) bottom water temperature (BWT) record from 16.0 Ma to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean, and compare it to existing BWT records. We show that BWTs in the Southern Ocean were ~8–10 °C during the middle Miocene greenhouse, and thus considerably warmer than today. Nonetheless, bottom water δ18O (calculated from foraminiferal δ18O and ∆47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset demonstrates that BWTs at Site 747 decreased by ~3–5 °C across the MMCT. This cooling preceded the stepped main increase in global ice volume, and appears to have been followed by a transient bottom water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Additional processes and feedbacks required for substantial ice growth may have contributed to the observed decoupling of Southern Ocean BWT and global ice volume.