Frank Schmieder

The Mid-Pleistocene climate transition as documented in the deep South Atlantic Ocean: initiation, interim state and terminal event

Abstract The Mid-Pleistocene transition (MPT) of the global climate system, initiated by a shift towards much larger northern hemisphere ice shields at around 920 ka and ending with predominance of 100 kyr ice age cyclicity since about 640 ka, is one of the fundamental enigmas in Quaternary climate evolution. Climate proxy records not exclusively linked to global ice volume are necessary to advance understanding of the MPT. Here we present a high-resolution Pleistocene magnetic susceptibility time series of 12 sediment cores from the subtropical South Atlantic essentially reflecting dissolution driven variations in carbonate accumulation controlled by changes in deep water circulation. In addition to characteristics known from δ18O records, the data sets reveal three remarkable features intimately related to the MPT: (1) an all-Pleistocene minimum of carbonate accumulation in the South Atlantic at 920 ka, (2) a MPT interim state of reduced carbonate deposition, indicating that the MPT period may have been a discrete state of the Pleistocene deep water circulation and climate system and (3) a terminal MPT event at around 540–530 ka documented in several peculiarities such as thick laminated layers of the giant diatom Ethmodiscus rex.

Paleonutrient and productivity records from the subarctic North Pacific for Pleistocene glacial terminations I to V

Our study addresses fundamental questions of the mode and timing of orbital and millennial-scale changes in the meridional overturning circulation (MOC) of the subarctic North Pacific. Particular concerns are the vertical mixing, the present and past abundance of nutrients in surface waters despite strong stratification, and the North Pacific-North Atlantic seesaw of oscillations in sea surface temperature (SST). We do this by generating and interpreting multiple records for glacial terminations I-V down two long piston cores, one each from the western and eastern subarctic Pacific. Chlorins and biogenic opal are proxies for surface water productivity; delta(13)C of epibenthic foraminifera is a record of deepwater ventilation; and the delta(13)C of N. pachyderma sin. is a tracer of nutrients in subsurface waters that extend up to the sea surface during times of vertical mixing. The degree of mixing is traced by pairing SST and delta(18)O records of planktic surface and subsurface (pycnocline) dwellers. Tight age control is deduced from a suite of age-calibrated (14)C plateau boundaries for Termination I and benthic delta(18)O and geomagnetic events for the last 800 ka. Carbon 14 paleoreservoir ages record the ages of surface and deep waters to uncover short-term changes in MOC over Termination I. We have defined a standard sequence of short-term productivity events for Termination I, also evident during terminations II to V and subsequent interglacials over the last 450 ka. The peak glacial regime of stable stratification and low productivity terminated, together with the end of ice rafting and melting, near 17 ka, similar to 2000 years after the onset of Termination I. Pulses of vertical mixing and incursion of warm surface waters from the subtropics followed. Convected young water masses finally penetrated down to 3600-m water depth at 17.0 to less than 14.5 ka, significantly improving bottom water ventilation through the late deglacial and earliest interglacial. Mixing with upwelled nutrients from the pycnocline induced short-term maxima in algal production of chlorins and biogenic opal near 17-15 and 15-12 ka, respectively. Deglacial meltwater incursions in the Aleutian Current and silica input from North American rivers also promoted East Pacific productivity after 15.5 ka. Productivity decreased during the late deglacial and early interglacial, coeval with an exceptional peak in CaCO(3) preservation caused by both low organic flux and well-ventilated deepwater. Subsequently, low salinity and cool surface waters and in turn, stratification were gradually restored. A second, opal-dominated productivity maximum marked the ends of interglacials. The deglacial pulses of vertical mixing around 17-11 ka imply an important contribution of the North Pacific to the coeval release of oceanic CO(2) into the atmosphere and support the east-west seesaw model of climate change.

Using Rock Magnetic Proxy Records for Orbital Tuning and Extended Time Series Analyses into the Super- and Sub-Milankovitch Bands

High-resolution rock magnetic proxy records of marine sediments, in particular magnetic susceptibility logs, delineate variations of sediment lithology and mirror climatic and oceanographic changes of different duration. Most commonly, Milankovitch cyclicity resulting from orbital forcing of carbonate dissolution and terrigenous sedimentation prevails. Extracted by bandpass filtering, these signal components can serve for multiple core correlation, cyclostratigraphic analyses and orbital tuning. Phase relations between astronomical obliquity and precession cycles and their equivalents in rock magnetic records depend on regional sedimentological settings. Two case studies are developed to demonstrate specific aims, strategies, strengths and restrictions of rock magnetic time series analyses and their extension into the super- and sub-Milankovitch bands. In the first example, twelve Pleistocene susceptibility records spanning the oligotrophic subtropical South Atlantic (SUSAS stack) were successfully tuned to obliquity and precession despite their low average sedimentation rates < 1 cm/kyr. Multiple bandpass filtering and evolutionary spectral analysis reveal two major base line shifts at around 0.95 and 0.6 Ma and various superstructures (e.g., amplitude modulations) of orbital proxy response. Compared to analogous analyses of an adapted astronomical target curve, converse residue patterns in the 41 kyr and 100 kyr bands indicate that Pleistocene ice age cycles were mainly triggered by obliquity before 1.25 Ma and from 1.05 to 0.7 Ma, while eccentricity modulation of precession predominated between 1.25 and 1.05 Ma and during the last 0.6 Ma. In the second example, eight susceptibility records from the western equatorial Atlantic Ceara Rise (CEARIS stack) were tuned to a lagged precession index signal. Subsequently, a high resolution core correlation scheme was established on basis of their coherent high-frequency (< 15 kyr) signal patterns. Basic harmonics and intermodulation frequencies of obliquity, and, predominately, precession resulting from nonlinear proxy response were detected by sub-Milankovitch spectral and bispectral analyses. Twin susceptibility peaks corresponding to a tropical double precession cycle appear even in the unfiltered records. Millennial signal variations (< 7 kyr) seem to coincide with Bond- and Dansgaard-Oeschger cycles.

Anomalous South Atlantic lithologies confirm global scale of unusual mid-Pleistocene climate excursion

Abstract Carbonate-rich nannofossil oozes are the typical sediments deposited under the low-productivity regime of the subtropical gyre in the South Atlantic. Surface productivity has remained virtually constant during the late Pleistocene [Schmiedl and Mackensen, Palaeogeogr. Palaeoclimatol. Palaeoecol. 130 (1997) 43–80], increasing slightly only during the most extreme glacial periods. Recently, the puzzling occurrence of thick monospecific diatom layers ( Ethmodiscus rex ) has been reported within the carbonaceous sequences of two sediment cores in the South Atlantic Gyre [Schmieder et al., Earth Planet. Sci. Lett. 179 (2000) 539–549]. The layers, dated 544–534 ka, are isochronous and record a period of increased organic carbon flux. Above them, at 530–524 ka, unusually high carbonate contents are recorded. This double feature of anomalous lithologies is unique in the sediment record of the last 1500 kyr and can be related to the terminal event of the mid-Pleistocene climate transition (MPT). The MPT is characterized by reduced thermohaline circulation and stronger carbonate dissolution [Schmieder et al., Earth Planet. Sci. Lett. 179 (2000) 539–549]. Reduced circulation and a more stratified water column may have trapped nutrients in deep nitrate pools and may have fostered favorable conditions for the development of deep-dwelling populations of E. rex . The onset of more turbid conditions led to the rapid settling of diatom populations, subsequently introduced nutrients to the surface layer, and improved calcite preservation, thus creating a biogenic silica–carbonate double peak. The carbonate peak is synchronous with an unusual mid-Pleistocene sapropel observed in the Mediterranean, which is attributed to a ‘massive odd monsoon’ over Central Africa [Rossignol-Strick et al., Nature 392 (1998) 269–272]. Since summer insolation was relatively low during this time the formation of this thick sapropel cannot be explained in terms of current understanding of astronomical forcing [Rossignol-Strick et al., Nature 392 (1998) 269–272]. A non-linear response to a 400-kyr low-frequency climatic cycle as discussed for the Mediterranean sapropel is not confirmed by data from our South Atlantic core records.

Terrigenous Flux in the Rio Grande Rise Area during the Past 1500 ka: Evidence of Deepwater Advection or Rapid Response to Continental Rainfall Patterns?

Surface sediment samples and three gravity cores from the eastern terrace of the Vema Channel, the western flank of the Rio Grande Rise, and the Brazilian continental slope were investigated for physical properties, grain size, and clay mineral composition. Discharge of the Rio Doce is responsible for kaolinite enrichments on the slope south of 20° and at intermediate depths of the Rio Grande Rise. The long-distance advection of kaolinite with North Atlantic Deep Water from lower latitudes is of minor importance as evidenced by low kaolinite/chlorite ratios on the Mid-Atlantic Ridge. Cyclic variations of kaolinite/chlorite ratios in all our cores, with maxima in interglacials, are attributed to low- and high-latitude forcing of paleoclimate on the Brazilian mainland and the related discharge of the Rio Doce. A long-term trend toward more arid and “glacial” conditions from 1500 ka to present is superimposed on the glacial-interglacial cyclicity.

Magnetic Signals in Plio-Pleistocene Sediments of the South Atlantic: Chronostratigraphic Usability and Paleoceanographic Implications

The origin of the magnetic signals used to build age models for marine sediments recovered in the framework of the long-term Quaternary South Atlantic research project SFB 261 is twofold. Conventional magnetostratigraphy makes use of well-dated polarity reversals of the Earth’s magnetic field recorded in the natural remanent magnetization (NRM) of the sediments. In addition, magnetic cyclostratigraphy has been successfully established as a very efficient dating tool for marine sediment sequences during recent years. In the oligotrophic South Atlantic, confirmation of orbital forcing of magnetic susceptibility records made it possible to establish high-resolution age models, by tuning the respective components to astronomical variations. A set of twelve individually tuned and well-correlated Pleistocene magnetic susceptibility records were stacked within the stratigraphic network SUSAS and can now be used as a correlation reference for other cores recovered in this region. The suitability of this target curve for age control is tested against paleomagnetic ages. Pattern correlation is possible for nine of ten selected cores recovered in the oligotrophic South Atlantic between 15°S and 35°S, but seems to be only partly successful for sediments from the Congo Basin. In the Pleistocene sequences, the magnetic age models provide further evidence for the simultaneous deposition of previously reported unusual diatom ooze layers between 23°S and 33°S at approximately 540 — 530 ka, at the end of the Mid-Pleistocene climate transition (MPT). The age models also indicate enhanced carbonate dissolution during the MPT interim state (920 - 640 ka). The concept of tuning magnetic susceptibility records to orbital variations is extended to the late Pliocene and reveals characteristics obviously related to the rearrangement of ocean circulation as Northern Hemisphere glaciation intensified. Enhanced carbonate preservation since approximately 3.0 Ma and the establishment of obliquity-driven dissolution cycles since about 2.5 Ma document increasing influx of North Atlantic Deep Water (NADW) into the subtropical South Atlantic. In a deep core from the Rio Grande Rise area, an abrupt change from red deep-sea clay to carbonaceous sediments is recorded at 2.73 Ma, exactly the time proposed for the major intensification of Northern Hemisphere glaciation.