Lucas J. Lourens

The Neogene Period

An Astronomically Tuned Neogene Time Scale (ATNTS2012) is presented, as an update of ATNTS2004 in GTS2004. The new scale is not fundamentally different from its predecessor and the numerical ages are identical or almost so. Astronomical tuning has in principle the potential of generating a stable Neogene time scale as a function of the accuracy of the La2004 astronomical solution used for both scales. Minor problems remain in the tuning of the Lower Miocene. In GTS2012 we will summarize what has been modified or added since the publication of ATNTS2004 for incorporation in its successor, ATNTS2012. Mammal biostratigraphy and its chronology are elaborated, and the regional Neogene stages of the Paratethys and New Zealand are briefy discussed. To keep changes to ATNTS2004 transparent we maintain its subdivision into headings as much as possible.

Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum

The Palaeocene/Eocene thermal maximum, ∼55 million years ago, was a brief period of widespread, extreme climatic warming, that was associated with massive atmospheric greenhouse gas input. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition. We show that sea surface temperatures near the North Pole increased from ∼18 °C to over 23 °C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the oceans bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations, but the absolute polar temperatures that we derive before, during and after the event are more than 10 °C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms—perhaps polar stratospheric clouds or hurricane-induced ocean mixing—to amplify early Palaeogene polar temperatures.

Astronomical pacing of late Palaeocene to early Eocene global warming events

At the boundary between the Palaeocene and Eocene epochs, about 55 million years ago, the Earth experienced a strong global warming event, the Palaeocene–Eocene thermal maximum. The leading hypothesis to explain the extreme greenhouse conditions prevalent during this period is the dissociation of 1,400 to 2,800 gigatonnes of methane from ocean clathrates, resulting in a large negative carbon isotope excursion and severe carbonate dissolution in marine sediments. Possible triggering mechanisms for this event include crossing a threshold temperature as the Earth warmed gradually, comet impact, explosive volcanism or ocean current reorganization and erosion at continental slopes, whereas orbital forcing has been excluded. Here we report a distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge, which we term the Elmo horizon. Using orbital tuning, we estimate deposition of the Elmo horizon at about 2 million years after the Palaeocene–Eocene thermal maximum. The Elmo horizon has similar geochemical and biotic characteristics as the Palaeocene–Eocene thermal maximum, but of smaller magnitude. It is coincident with carbon isotope depletion events in other ocean basins, suggesting that it represents a second global thermal maximum. We show that both events correspond to maxima in the ∼405-kyr and ∼100-kyr eccentricity cycles that post-date prolonged minima in the 2.25-Myr eccentricity cycle, implying that they are indeed astronomically paced.

Extending the astronomical ( polarity) time scale into the Miocene

An astronomical time scale is presented for the late Miocene based on the correlation of characteristic sedimentary cycle patterns in marine sections in the Mediterranean to the 65”N summer insolation curve of La90 [ 1,2] with present-day values for the dynamical ellipticity of the Earth and tidd dissipation by the moon. This correlation yields ages for all sedimentary cycles and hence also for the recorded polarity reversals, and planktonic foraminiferal and dinoflagellate events. The Tortonian/Messinian (T/M) boundary placed at the first regular occurrence of the Globorofuliu conomiozea group in the Mediterranean is dated at 7.24 Ma. The duration of the Messinian is estimated at 1.91 Myr because the Miocene/Pliocene boundary has been dated previously at 5.33 Ma [3]. The new time scale is confirmed by “OAr/ 3gAr ages of volcanic beds and by the number of sedimentary cycles in the younger part of the Mediterranean Messinian.

Temporal variability in the northern Arabian Sea oxygen minimum zone (OMZ) during the last 225,000 years

The northern Arabian Sea is one of the few regions in the open ocean where thermocline water is severely depleted in oxygen. The intensity of this oxygen minimum zone (OMZ) has been reconstructed over the past 225,000 years using proxies for surface water productivity, water column denitrification, winter mixing, and the aragonite compensation depth (ACD). Changes in OMZ intensity occurred on orbital and suborbital timescales. Lowest O2 levels correlate with productivity maxima and shallow winter mixing. Precession-related productivity maxima lag early summer insolation maxima by ∼6 kyr, which we attribute to a prolonged summer monsoon season related to higher insolation at the end of the summer. Periods with a weakened or even non-existent OMZ are characterized by low productivity conditions and deep winter mixing attributed to strong and cold winter monsoonal winds. The timing of deep winter mixing events corresponds with that of periods of climatic cooling in the North Atlantic region.

Long-period astronomical forcing of mammal turnover

Mammals are among the fastest-radiating groups, being characterized by a mean species lifespan of the order of 2.5 million years (Myr). The basis for this characteristic timescale of origination, extinction and turnover is not well understood. Various studies have invoked climate change to explain mammalian species turnover, but other studies have either challenged or only partly confirmed the climate–turnover hypothesis. Here we use an exceptionally long (24.5–2.5 Myr ago), dense, and well-dated terrestrial record of rodent lineages from central Spain, and show the existence of turnover cycles with periods of 2.4–2.5 and 1.0 Myr. We link these cycles to low-frequency modulations of Milankovitch oscillations, and show that pulses of turnover occur at minima of the 2.37-Myr eccentricity cycle and nodes of the 1.2-Myr obliquity cycle. Because obliquity nodes and eccentricity minima are associated with ice sheet expansion and cooling and affect regional precipitation, we infer that long-period astronomical climate forcing is a major determinant of species turnover in small mammals and probably other groups as well.

Making sense of palaeoclimate sensitivity

Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in K W−1 m2) of 0.3–1.9 or 0.6–1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2–4.8 K per doubling of atmospheric CO2, which agrees with IPCC estimates.

The response of the African summer monsoon to remote and local forcing due to precession and obliquity

In this paper, we examine the orbital signal in Earth’s climate with a coupled model of intermediate complexity (ECBilt). The orbital influence on climate is studied by isolating the obliquity and precession signal in several time-slice experiments. Focus is on monsoonal systems with emphasis on the African summer monsoon. The model shows that both the precession and the obliquity signal in the African summer monsoon consists of an intensified precipitation maximum and further northward extension during minimum precession and maximum obliquity than during maximum precession and minimum obliquity. In contrast to obliquity, precession also influences the seasonal timing of the occurrence of the maximum precipitation. The response of the African monsoon to orbital-induced insolation forcing can be divided into a response to insolation forcing at high northern latitudes and a response to insolation forcing at low latitudes, whereby the former dominates. The results also indicate that the amplitude of the precipitation response to obliquity depends on precession, while the precipitation response to precession is independent of obliquity. Our model experiments provide an explanation for the precession and obliquity signals in sedimentary records of the Mediterranean (e.g., Lourens et al. [Paleoceanography 11 (1996) 391, Nature 409 (2001) 1029]), through monsoon-induced variations in Nile river outflow and northern Africa aridity. D 2003 Elsevier Science B.V. All rights reserved.

Sulphidic Mediterranean surface waters during Pliocene sapropel formation

Sapropels—organic-matter rich layers—are common in Neogene sediments of the eastern Mediterranean Sea. The formation of these layers has been attributed to climate-related increases in organic-matter production and increased organic-matter preservation due to oxygen depletion in more stagnant bottom waters,. Here we report that eastern Mediterranean Pliocene sapropels contain molecular fossils of a compound (isorenieratene) known to be synthesized by photosynthetic green sulphur bacteria, suggesting that sulphidic (euxinic)—and therefore anoxic—conditions prevailed in the photic zone of the water column. These sapropels also have a high trace-metal content, which is probably due to the efficient scavenging of these metals by precipitating sulphides in a euxinic water column. The abundance and sulphur-isotope composition of pyrite are consistent with iron sulphide formation in the water column. We conclude that basin-wide water-column euxinia occurred over substantial periods during Pliocene sapropel formation in the eastern Mediterranean Sea, and that the ultimate degradation of the increased organic-matter production was strongly influential in generating and sustaining the euxinic conditions.

The formation of Pliocene sapropels and carbonate cycles in the Mediterranean: diagenesis, dilution, and productivity

High-resolution micropaleontological (planktonic foraminifera and calcareous nannofossils) and geochemical (stable isotopes, organic carbon, Fe, P, S, Ca, Ba, Mn, and Al) records are presented for the first sapropel-containing carbonate cycle in the Pliocene of Sicily. The carbonate cycle is characterized by a gray to white to beige to white color layering typical of the marls of the Trubi formation. A faintly laminated sapropel is intercalated in the gray-colored bed of the carbonate cycle. CaCO3 content varies from 40% in the beige to 45-50% in the white layers. Lowest CaCO3 content of 25–30% is found in the gray layer and sapropel. Variations in carbonate and organic matter percentages can best be explained by changes in paleoproductivity rather than by variations in dilution and dissolution. Total productivity was highest during deposition of the gray layer and sapropel, as indicated by high organic carbon and Ba contents and high abundance of Globorotalia puncticulata. Carbonate production reached its highest values, however, during deposition of the white layers, as evidenced by enhanced abundances of planktonic foraminifera and nannofossils. The low carbonate content in the gray layer and sapropel is explained in terms of a collapse in carbonate production caused by extreme changes in the physical and biochemical properties of the water column, which in turn resulted in siliceous plankton and opportunistic foraminifers such as Globorotalia puncticulata outcompeting most calcareous organisms. The beige layer represents a low-productivity environment similar to the present-day eastern Mediterranean basin. Several mechanisms have previously been proposed to explain variations in productivity in the eastern Mediterranean. Both sapropels and gray layers were deposited at times when perihelion occurred in northern hemisphere summer. We envisage that the increase in seasonal contrast resulting from this orbital configuration enhanced winter mixing and stabilization of the water column during summer, both leading to favorable conditions for intensification of the spring bloom. In addition, a decrease in excess evaporation, as can be deduced from the δ18O record, led to shoaling of the pycnocline and reduced circulation, thus enhancing the availability of nutrients in the photic zone. Finally, enhanced precipitation and associated runoff should have caused an increase in river-borne nutrients.