Thomas Frederichs

Arctic Ocean evidence for late Quaternary initiation of northern Eurasian ice sheets

A high-resolution multiparameter stratigraphy allows the identification of late Quaternary glacial and interglacial cycles in a central Arctic Ocean sediment core. Distinct sandy layers in the upper part of the otherwise fine-grained sediment core from the Lomonosov Ridge (lat 87.5°N) correlate to four major glacials since ca. 0.7 Ma. The composition of these ice-rafted terrigenous sediments points to a glaciated northern Siberia as the main source. In contrast, lithic carbonates derived from North America are also present in older sediments and indicate a northern North American glaciation since at least 2.8 Ma. We conclude that large-scale northern Siberian glaciation began much later than other Northern Hemisphere ice sheets.

Effects of Arctic freshwater forcing on thermohaline circulation during the Pleistocene

In this paper, we make use of calcium carbonate-bearing sediment sequences in the Arctic-Atlantic gateway to produce a standard reference climate record for the Arctic Ocean. We present a continuous and exceptionally well-dated marine sediment record documenting the strong imprint of Arctic freshwater pulses on Earths climate system throughout the past 0.8 m.y. Planktic foraminiferal oxygen and carbon isotope data reveal that freshwater plumes released from collapsing circum-Arctic ice sheets and ice-dammed lakes were advected to the Arctic Ocean gateway on sub-Milankovitch time scales. Given the close correspondence of Arctic freshwater pulses and strength of North Atlantic Deep Water formation as inferred from carbon isotope data, we conclude that freshening of the Arctic Ocean influenced thermohaline circulation more frequently than previously recognized. Our data show that it is critical for coupled atmosphere-ice-ocean general circulation models aimed at unraveling climate forcing factors during the Pleistocene to consider Arctic freshwater as a crucial forcing factor.

The Magnetic View on the Marine Paleoenvironment: Parameters, Techniques and Potentials of Rock Magnetic Studies as a Key to Paleoclimatic and Paleoceanographic Changes

The eminent potential of Environmental Magnetism analytical techniques to delineate depositional regimes and climatic changes in the marine realm is reviewed and illustrated with results of three individual studies of sediment series from the South Atlantic Ocean. Rock magnetic properties related to the mineralogy, concentration, domain state and hence grain-size of the magnetic mineral assemblage are explained on grounds of physical principles and discussed as proxy parameters for terrigenous particle fluxes, bioproductivity and diagenetic redox conditions. With cluster analysis of rock magnetic parameters determined for a large collection of surface samples, the regional characteristics of recent depositional environments in the equatorial South Atlantic are established. Notably, the different input mechanisms of terrigenous material via fluvial transport by the Amazon and Congo Rivers at the African and South American continental margins are as clearly identified as the eolian transport from the Sahara and Sahel Zone into the central and eastern equatorial South Atlantic. Based on a detailed susceptibility log and measurements of various laboratory remanences, high-coercive hematite components and different magnetite grain-size fractions could quantitatively be discriminated in a late Quaternary sediment sequence from the central equatorial Atlantic. The data sets allow to assess variations in eolian influx from the Saharan dust plume and several redox events during the last 400 kyr can be recognized. While biogenic magnetite is generally of minor importance in pelagic deposits, it may completely dominate the sediment magnetic properties in high productive areas. An intense primary biologic productivity in surface waters of the Benguela upwelling center supplies a high flux of organic matter to the sea floor at the continental slope off Namibia and causes reducing conditions in the sediment column. Resulting strong diagenetic effects on the biomagnetic mineral component are traced in detail by high-resolution rock magnetic analyses and transmission electron micrographs.

Sedimentation rates in the Makarov Basin, central Arctic Ocean: A paleomagnetic and rock magnetic approach

Three long sediment cores from the Makarov Basin have been subjected to detailed paleomagnetic and rock magnetic analyses. Investigated sediments are dominated by normal polarity including short reversal excursions, indicating that most of the sediments are of Brunhes age. In general, the recovered sediments show only low to moderate variability in concentration and grain size of the remanence-carrying minerals. Estimations of relative paleointensity variations yielded a well-documented succession of pronounced lows and highs that could be correlated to published reference curves. However, together with five accelerator mass spectrometry C-14 ages and an incomplete Be-10 record, still two different interpretations of the paleomagnetic data are possible, with long-term sedimentation rates of either 1.3 or 4 cm kyr(-1) However, both models implicate highly variable sedimentation rates of up to 10 cm kyr(-1), and abrupt changes in rock magnetic parameters might even indicate several hiatuses.

Methane-driven late Pleistocene δ13C minima and overflow reversals in the southwestern Greenland Sea

A core transect across the southwestern Greenland Sea reveals coeval events of extremely negative planktic and benthic δ 1 3 C excursions between 40 and 87 ka. The most pronounced event, event 1, began at peak Dansgaard-Oeschger stadial 22 (85 ka) with a duration of 18 k.y. During this episode, incursions of Atlantic Intermediate Water caused a bottom-water warming of up to 8 °C. The amplitude, timing, and geographic pattern of the δ 1 3 C events suggest that this bottom-water warming triggered clathrate instability along the East Greenland slope and a methane-induced depletion of δ 1 3 C D I C (DIC-dissolved inorganic carbon). Since δ 1 3 C event 1 matches a major peak in atmospheric CH 4 concentration, this clathrate destabilization may have contributed to the rise in atmospheric CH 4 and thus to climate warming over marine isotope stage 5.1.

Automated paleomagnetic and rock magnetic data acquisition with an in‐line horizontal “2G” system

Todays paleomagnetic and magnetic proxy studies involve processing of large sample collections while simultaneously demanding high quality data and high reproducibility. Here we describe a fully automated interface based on a commercial horizontal pass-through “2G” DC-SQUID magnetometer. This system is operational at the universities of Bremen (Germany) and Utrecht (Netherlands) since 1998 and 2006, respectively, while a system is currently being built at NGU Trondheim (Norway). The magnetometers are equipped with “in-line” alternating field (AF) demagnetization, a direct-current bias field coil along the coaxial AF demagnetization coil for the acquisition of anhysteretic remanent magnetization (ARM) and a long pulse-field coil for the acquisition of isothermal remanent magnetization (IRM). Samples are contained in dedicated low magnetization perspex holders that are manipulated by a pneumatic pick-and-place-unit. Upon desire samples can be measured in several positions considerably enhancing data quality in particular for magnetically weak samples. In the Bremen system, the peak of the IRM pulse fields is actively measured which reduces the discrepancy between the set field and the field that is actually applied. Techniques for quantifying and removing gyroremanent overprints and for measuring the viscosity of IRM further extend the range of applications of the system. Typically c. 300 paleomagnetic samples can be AF demagnetized per week (15 levels) in the three-position protocol. The versatility of the system is illustrated by several examples of paleomagnetic and rock magnetic data processing.

Cyclic magnetite dissolution in Pleistocene sediments of the abyssal northwest Pacific Ocean: Evidence for glacial oxygen depletion and carbon trapping

The carbonate-free abyss of the North Pacific defies most paleoceanographic proxy methods and hence remains a “blank spot” in ocean and climate history. Paleomagnetic and rock magnetic, geochemical, and sedimentological methods were combined to date and analyze seven middle to late Pleistocene northwest Pacific sediment cores from water depths of 5100 to 5700 m. Besides largely coherent tephra layers, the most striking features of these records are nearly magnetite-free zones corresponding to glacial marine isotope stages (MISs) 22, 12, 10, 8, 6, and 2. Magnetite depletion is correlated with organic carbon and quartz content and anticorrelated with biogenic barite and opal content. Within interglacial sections and mid-Pleistocene transition glacial stages MIS 20, 18, 16, and 14, magnetite fractions of detrital, volcanic, and bacterial origin are all well preserved. Such alternating successions of magnetic iron mineral preservation and depletion are known from sapropel-marl cycles, which accumulated under periodically changing bottom water oxygen and redox conditions. In the open central northwest Pacific Ocean, the only conceivable mechanism to cause such abrupt change is a modified glacial bottom water circulation. During all major glaciations since MIS 12, oxygen-depleted Antarctic Bottom Water (AABW)-sourced bottom water seems to have crept into the abyssal northwest Pacific below ~5000 m depth, thereby changing redox conditions in the sediment, trapping and preserving dissolved and particulate organic matter and, in consequence, reducing and dissolving both, biogenic and detrital magnetite. At deglaciation, a downward progressing oxidation front apparently remineralized and released these sedimentary carbon reservoirs without replenishing the magnetite losses.

Limited grounding-line advance onto the West Antarctic continental shelf in the easternmost Amundsen Sea Embayment during the last glacial period

Precise knowledge about the extent of the West Antarctic Ice Sheet (WAIS) at the Last Glacial Maximum (LGM; c. 26.5–19 cal. ka BP) is important in order to 1) improve paleo-ice sheet reconstructions, 2) provide a robust empirical framework for calibrating paleo-ice sheet models, and 3) locate potential shelf refugia for Antarctic benthos during the last glacial period. However, reliable reconstructions are still lacking for many WAIS sectors, particularly for key areas on the outer continental shelf, where the LGM-ice sheet is assumed to have terminated. In many areas of the outer continental shelf around Antarctica, direct geological data for the presence or absence of grounded ice during the LGM is lacking because of post-LGM iceberg scouring. This also applies to most of the outer continental shelf in the Amundsen Sea. Here we present detailed marine geophysical and new geological data documenting a sequence of glaciomarine sediments up to ~12 m thick within the deep outer portion of Abbot Trough, a palaeo-ice stream trough on the outer shelf of the Amundsen Sea Embayment. The upper 2–3 meters of this sediment drape contain calcareous foraminifera of Holocene and (pre-)LGM age and, in combination with palaeomagnetic age constraints, indicate that continuous glaciomarine deposition persisted here since well before the LGM, possibly even since the last interglacial period. Our data therefore indicate that the LGM grounding line, whose exact location was previously uncertain, did not reach the shelf edge everywhere in the Amundsen Sea. The LGM grounding line position coincides with the crest of a distinct grounding-zone wedge ~100 km inland from the continental shelf edge. Thus, an area of ≥6000 km2 remained free of grounded ice through the last glacial cycle, requiring the LGM grounding line position to be re-located in this sector, and suggesting a new site at which Antarctic shelf benthos may have survived the last glacial period.

Evidence for a palaeo-subglacial lake on the Antarctic continental shelf.

Subglacial lakes are widespread beneath the Antarctic Ice Sheet but their control on ice-sheet dynamics and their ability to harbour life remain poorly characterized. Here we present evidence for a palaeo-subglacial lake on the Antarctic continental shelf. A distinct sediment facies recovered from a bedrock basin in Pine Island Bay indicates deposition within a low-energy lake environment. Diffusive-advection modelling demonstrates that low chloride concentrations in the pore water of the corresponding sediments can only be explained by initial deposition of this facies in a freshwater setting. These observations indicate that an active subglacial meltwater network, similar to that observed beneath the extant ice sheet, was also active during the last glacial period. It also provides a new framework for refining the exploration of these unique environments.

Late Lutetian Thermal Maximum—Crossing a Thermal Threshold in Earth's Climate System?

Recognizing and deciphering transient global warming events triggered by massive release of carbon into Earths ocean-atmosphere climate system in the past are important for understanding climate under elevated pCO 2 conditions. Here we present new high-resolution geochemical records including benthic foraminiferal stable isotope data with clear evidence of a short-lived (30 kyr) warming event at 41.52 Ma. The event occurs in the late Lutetian within magnetochron C19r and is characterized by a ∼2°C warming of the deep ocean in the southern South Atlantic. The magnitudes of the carbon and oxygen isotope excursions of the Late Lutetian Thermal Maximum are comparable to the H2 event (53.6 Ma) suggesting a similar response of the climate system to carbon cycle perturbations even in an already relatively cooler climate several million years after the Early Eocene Climate Optimum. Coincidence of the event with exceptionally high insolation values in the Northern Hemisphere at 41.52 Ma might indicate that Earths climate system has a thermal threshold. When this tipping point is crossed, rapid positive feedback mechanisms potentially trigger transient global warming. The orbital configuration in this case could have caused prolonged warm and dry season leading to a massive release of terrestrial carbon into the ocean-atmosphere system initiating environmental change.