Milankovitch-paced erosion in the southern Central Andes

It has long been hypothesized that climate can modify both the pattern and magnitude of erosion in mountainous landscapes, thereby controlling morphology, rates of deformation, and potentially modulating global carbon and nutrient cycles through weathering feedbacks. Although conceptually appealing, geologic evidence for a direct climatic control on erosion has remained ambiguous owing to a lack of high-resolution, long-term terrestrial records and suitable field sites. Here we provide direct terrestrial field evidence for long-term synchrony between erosion rates and Milankovitch-driven, 400-kyr eccentricity cycles using a Plio-Pleistocene cosmogenic radionuclide paleo-erosion rate record from the southern Central Andes. The observed climate-erosion coupling across multiple orbital cycles, when combined with results from the intermediate complexity climate model CLIMBER-2, are consistent with the hypothesis that relatively modest fluctuations in precipitation can cause synchronous and nonlinear responses in erosion rates as landscapes adjust to ever-evolving hydrologic boundary conditions imposed by oscillating climate regimes

Modeling orbital induced variations in circum-Mediterranean climate

The climate of the Earth varies both irregular and (quasi)periodic over a broad range of time-scales. The variations with periods of some ten-thousands of years are caused by variations in the shape of the orbit of the Earth and the orientation of the rotation axis of the Earth. These variations strongly affect the strength and the spatial and seasonal pattern of the insolation received by the Earth. This leads to climatic oscillations which are often recorded in sedimentary archives. One example is the deposition of sapropels (organic-rich black layers) which are ubiquitously present in the deeper parts of the Mediterranean Sea throughout at least the last 13 million years. The proxy data related to sapropels clearly point to a precession (date of perihelion) dominated oscillation in the climate system. However, also an obliquity (tilt of the rotation axis of the Earth) signal is found in the sapropel record, despite the weak obliquity signal in the insolation at low latitudes. The paleoclimatic origin of sapropels is not fully understood. The most used hypothesis is that sapropels are deposited when the discharge of the river Nile (which is determined by the strength of the African summer monsoon) is high. However, the role of discharge from European rivers and the role of precipitation over the Mediterranean Sea is not clear. Another uncertainty is the timing of the deposition of sapropels, i.e., the youngest sapropel lags precession by about 3,000 years and it is not clear whether this is the case for all sapropels. In this research the orbital signals in the African summer monsoon and in the European climate are studied with climate models. The results show that a strong precession signal is present in the discharge of the river Nile which is in agreement with the hypothesis. Furthermore, an obliquity signal exists in the discharge which is caused by remote influences from high latitudes. In addition to the orbital signals in the discharge of the river Nile, the river discharge from the northern borderlands and the precipitation over and around the Mediterranean Sea also show a precession as well as an obliquity signal. The precession signal in the precipitation is in agreement with a pollen record from Greece which is also described in this thesis. The model results show no clear indications for a lagged response of the circum-Mediterranean climate system to precession. However, a clear (vegetation induced) lag was found in the circulation in the Atlantic Ocean. It is not fully excluded that a similar mechanism can also cause a lag in the deposition of sapropels in the Mediterranean Sea. In order to determine which influence (from the south or from the north) is most important for the circulation in the Mediterranean Sea a model for the Mediterranean Sea is used. It turns out that the influence of increased Nile discharge is strictly located to the Eastern coast while increased discharge from European rivers and increased precipitation cause lower salinity over the entire Mediterranean Sea, especially over locations which are important for the deep circulation (i.e., where deep water is formed). The main conclusion drawn from this research is that model results indicate that the influence of the river Nile on the circulation in the Mediterranean Sea is smaller than thought before and that the deposition of sapropels could also be caused by changes in the hydrological cycle over the northern borderlands and over the Mediterranean Sea

(Table 1) Age control points of ODP Site 160-968

The astronomical timescale of the Eastern Mediterranean Plio-Pleistocene builds on tuning of sapropel layers to Northern Hemisphere summer insolation maxima. A 3000-year precession lag has become instrumental in the tuning procedure as radiocarbon dating revealed that the midpoint of the youngest sapropel, S1, in the early Holocene occurred approximately 3000 years after the insolation maximum. The origin of the time lag remains elusive, however, because sapropels are generally linked to maximum African monsoon intensities and transient climate modeling results indicate an in-phase behavior of the African monsoon relative to precession forcing. Here we present new high-resolution records of bulk sediment geochemistry and benthic foraminiferal oxygen isotopes from ODP Site 968 in the Eastern Mediterranean. We show that the 3000-year precession time lag of the sapropel midpoints is consistent with (1) the global marine isotope chronology, (2) maximum (monsoonal) precipitation conditions in the Mediterranean region and China derived from radiometrically dated speleothem records, and (3) maximum atmospheric methane concentrations in Antarctica ice cores. We show that the time lag relates to the occurrence of precession-paced North Atlantic cold events, which systematically delayed the onset of strong boreal summer monsoon intensity. Our findings may also explain a non-stationary behavior of the African monsoon over the past 3 million years due to more frequent and intensive cold events in the Late Pleistocene

The precession phase of the boreal summer monsoon as viewed from the eastern Mediterranean (ODP Site 968)

The astronomical timescale of the Eastern Mediterranean Plio–Pleistocene builds on tuning of sapropel layers to Northern Hemisphere summer insolation maxima. A 3000-year precession lag has become instrumental in the tuning procedure as radiocarbon dating revealed that the midpoint of the youngest sapropel, S1, in the early Holocene occurred approximately 3000 years after the insolation maximum. The origin of the time lag remains elusive, however, because sapropels are generally linked to maximum African monsoon intensities and transient climate modeling results indicate an in-phase behavior of the African monsoon relative to precession forcing. Here we present new high-resolution records of bulk sediment geochemistry and benthic foraminiferal oxygen isotopes from ODP Site 968 in the Eastern Mediterranean. We show that the 3000-year precession time lag of the sapropel midpoints is consistent with (1) the global marine isotope chronology, (2) maximum (monsoonal) precipitation conditions in the Mediterranean region and China derived from radiometrically dated speleothem records, and (3) maximum atmospheric methane concentrations in Antarctica ice cores. We show that the time lag relates to the occurrence of precession-paced North Atlantic cold events, which systematically delayed the onset of strong boreal summer monsoon intensity. Our findings may also explain a non-stationary behavior of the African monsoon over the past 3 million years due to more frequent and intensive cold events in the Late Pleistocene

Obliquity forcing of low-latitude climate

The influence of obliquity, the tilt of the Earth's rotational axis, on incoming solar radiation at low latitudes is small, yet many tropical and subtropical palaeoclimate records reveal a clear obliquity signal. Several mechanisms have been proposed to explain this signal, such as the remote influence of high-latitude glacials, the remote effect of insolation changes at mid- to high latitudes independent of glacial cyclicity, shifts in the latitudinal extent of the tropics, and changes in latitudinal insolation gradients. Using a sophisticated coupled ocean–atmosphere global climate model, EC-Earth, without dynamical ice sheets, we performed two idealized experiments of obliquity extremes. Our results show that obliquity-induced changes in tropical climate can occur without high-latitude ice sheet fluctuations. Furthermore, the tropical circulation changes are consistent with obliquity-induced changes in the cross-equatorial insolation gradient, suggesting that this gradient may be used to explain obliquity signals in low-latitude palaeoclimate records instead of the classical 65° N summer insolation curve

Precession and obliquity forcing of the freshwater budget over the Mediterranean

There is strong proxy and model evidence of precession- and obliquity-induced changes in the freshwater budget over the Mediterranean Sea and its borderlands, yet explanations for these changes vary greatly. We investigate the separate precession and obliquity forcing of the freshwater budget over the Mediterranean using a high-resolution coupled climate model, EC-Earth. At times of enhanced insolation seasonality, i.e. minimum precession and maximum obliquity, the area was wetter and the Mediterranean Sea surface was less saline. The latter has been attributed to increased runoff from the south as a consequence of a strengthened North African monsoon, as well as to increased precipitation over the Mediterranean Sea itself. Our results show that both mechanisms play a role in changing the freshwater budget. Increased monsoon runoff occurs in summer during times of enhanced insolation seasonality, especially minimum precession, while increased precipitation is important in winter for both precession and obliquity. We relate changes in winter precipitation to changes in the air-sea temperature difference and subsequently, convective precipitation. The freshening in the minimum precession and maximum obliquity experiments has a strong effect on Mediterranean sea surface salinity and mixed layer depth, thereby likely influencing deep sea circulation and sedimentation at the ocean bottom

Precession phasing offset between Indian summer monsoon and Arabian Sea productivity linked to changes in Atlantic overturning circulation

Results from transient climate modeling experiments indicate an in-phase relationship between insolation forcing and Indian summer monsoonal precipitation. This is in contrast to high-resolution radioisotopically dated speleothem oxygen isotope (delta O-18) records of China, which showed that East Asian Monsoon maxima lag Northern Hemisphere peak summer insolation by similar to 2,700 years, while an approximately 8,000-year time lag was derived from late Pleistocene records of Arabian Sea sediments. Here, we evaluate the precession phase of the Arabian Sea signal by comparing a new high-resolution productivity and oxygen minimum zone (OMZ) intensity record from the Arabian Sea over the past 450,000 years with the results of a transient climate modeling experiment that includes glacial-bound ice volume variations. The well established tuning technique between radioisotopically dated North Atlantic cold events and the occurrence of deep-dwelling planktonic foraminifera in the Arabian Sea for the last glacial cycle was used to extend the Arabian Sea chronology, independent of orbital tuning. Cross-spectral analysis over the last 224,000 years reveals that Arabian Sea productivity maxima lag precession minima by similar to 6,900 +/- 200 years, i.e., in close agreement with previous reconstructions. Also our climate modeling simulations are in accord with previous studies indicating an in-phase relationship between precession minima and maximum summer monsoon intensity. We argue that the summer monsoon is most likely not the main driver of changes in Arabian Sea biological productivity and OMZ intensity at the precession frequency band, but that changes in the intensity of the Atlantic meridional overturning circulation (AMOC) have played the prominent role in controlling the nutrient delivery into the euphotic layer of the northern Indian Ocean, and hence the amount of primary productivity and intensity of the oxygen minimum zone in the Arabian Sea. Such a mechanism explains the large precession-related time lag between minimum precession and maximum productivity and OMZ conditions in the Arabian Sea, since intensified AMOC occurred during precession maxima

Cenozoic global ice volume and temperature simulations over the past 40 million years

Variations in global ice volume and temperature over the Cenozoic era have been investigated with a set of one-dimensional (1-D) ice-sheet models. Simulations include three ice sheets representing glaciation in the Northern Hemisphere, i.e. in Eurasia, North America and Greenland, and two separate ice sheets for Antarctic glaciation. The continental mean Northern Hemisphere surface-air temperature has been derived through an inverse procedure from observed benthic d18O records. These data have yielded a mutually consistent and continuous record of temperature, global ice volume and benthic d18O over the past 35 Ma. The simple 1-D model shows good agreement with a comprehensive 3-D ice-sheet model for the past 3 Ma. On average, differences are only 1.0°C for temperature and 6.2 m for sea level. Most notably, over the 35 Ma period, the reconstructed ice volume–temperature sensitivity shows a transition from a climate controlled by Southern Hemisphere ice sheets to one controlled by Northern Hemisphere ice sheets. Although the transient behaviour is important, equilibrium experiments show that the relationship between temperature and sea level is linear and symmetric, providing limited evidence for hysteresis. Furthermore, the results show a good comparison with other simulations of Antarctic ice volume and observed sea level

Obliquity forcing of low-latitude climate

The influence of obliquity, the tilt of the Earth's rotational axis, on incoming solar radiation at low latitudes is small, yet many tropical and subtropical palaeoclimate records reveal a clear obliquity signal. Several mechanisms have been proposed to explain this signal, such as the remote influence of high-latitude glacials, the remote effect of insolation changes at mid- to high latitudes independent of glacial cyclicity, shifts in the latitudinal extent of the tropics, and changes in latitudinal insolation gradients. Using a sophisticated coupled ocean–atmosphere global climate model, EC-Earth, without dynamical ice sheets, we performed two idealized experiments of obliquity extremes. Our results show that obliquity-induced changes in tropical climate can occur without high-latitude ice sheet fluctuations. Furthermore, the tropical circulation changes are consistent with obliquity-induced changes in the cross-equatorial insolation gradient, suggesting that this gradient may be used to explain obliquity signals in low-latitude palaeoclimate records instead of the classical 65° N summer insolation curve

Simulation of northern hemispheric, southern hemispheric and global temperature over the past 5 million years

Model output of the intermediate complexity climate model CLIMBER-2 over the past 5 million years. The simulations were forced with insolation data (O), insolation and land ice data (OI), insolation and carbon dioxide data (OC) and with insolation, land ice and carbon dioxide data (OIC). Sheet 1 contains the main results: northern hemispheric (30-90 deg N), southern hemispheric (30-90 deg S) and global temperatures. Sheet 2 contains the land ice and carbon dioxide forcing in terms of globally averaged radiative forcing. Details are given in the publication. More information or data can be obtained by contacting L.B. Stap ([email protected])