Helge W. Arz’s research while affiliated with Leibniz Institute for Baltic Sea Research and other places

Millennial-scale variations in the strength and position of the Antarctic Circumpolar Current exert considerable influence on the global meridional overturning circulation and the ocean carbon cycle. The mechanistic understanding of these variations is still incomplete, partly due to the scarcity of sediment records covering multiple glacial-interglacial cycles with millennial-scale resolution. Here, we present high-resolution current strength and sea surface temperature records covering the past 790,000 years from the Cape Horn Current as part of the subantarctic Antarctic Circumpolar Current system, flowing along the Chilean margin. Both temperature and current velocity data document persistent millennial-scale climate variability throughout the last eight glacial periods with stronger current flow and warmer sea surface temperatures coinciding with Antarctic warm intervals. These Southern Hemisphere changes are linked to North Atlantic millennial-scale climate fluctuations, plausibly involving changes in the Atlantic thermohaline circulation. The variations in the Antarctic Circumpolar Current system are associated with atmospheric CO2 changes, suggesting a mechanistic link through the Southern Ocean carbon cycle.

Little is known about the genetic diversity and stability of natural populations over millennial time scales, although the current biodiversity crisis calls for heightened understanding. Marine phytoplankton, the primary producers forming the basis of food webs in the oceans, play a pivotal role in maintaining marine ecosystems health and serve as indicators of environmental change. This study examines the genetic diversity and shifts in allelic composition in the diatom species Skeletonema marinoi over ~ 8000 years in the Baltic Sea by analyzing chloroplast and mitochondrial genomes. Ancient environmental DNA (aeDNA) from sediment cores demonstrates stability and resilience of genetic composition and diversity of this species across millennia in the context of major climate events. Accelerated change in allelic composition is observed from historical periods onwards, coinciding with times of intensifying human activity, like the Roman Empire, the Viking Age, and the Hanseatic Age, suggesting that anthropogenic stressors have profoundly impacted this species for the last two millennia. The data indicate a very high natural stability and resilience of the genomic composition of the species and underscore the importance of uncovering genomic disruptions caused by human impact on organisms, even those not directly exploited, to better predict and manage future biodiversity.

Recent intensification of the Southern Hemisphere Westerlies has resulted in important changes to ocean circulation, Antarctic ice shelf stability and precipitation regimes in the continents abutting the Southern Ocean. Efforts to resolve the natural behaviour of the Westerlies over sub-millennial to millennial-timescales are critical to anticipating future changes with continued 21st Century warming. Here we present an ~11,000 year diatom-inferred sea salt aerosol and multiproxy geochemical record preserved in lake sediments from Cape Horn (56°S) which documents warm conditions and stronger-than-present Westerlies in the Early Holocene (10 000–7500 calibrated years before present) at this site. Combined with other regional records, we demonstrate that the Westerlies were poleward of their current position during the Early Holocene. This poleward migration of the Southern Hemisphere Westerlies in response to peak Holocene warmth provides an analogue for future warming and greater impacts on the southern high latitudes and global climate in the coming decades.

Three long sediment cores recovered from the SE Pacific were subjected to a comprehensive magnetostratigraphic investigation. According to the partly preliminary age models obtained, the cores reach 48, 140, and 482 ka back in time, respectively. At two sites (PC02 and PC03 at ∼46°S) sedimentation rates are highly variable, ranging from about 2 to 100 cmka⁻¹, whereas sedimentation rates are significantly lower at the southernmost site (PC04, ∼51°S), ranging from about 1 to 20 cmka⁻¹. All three cores provide evidence for the Laschamps geomagnetic excursion at 41 ka. However, the excursion was recorded during times of low sedimentation rates and thus, only little detail on geomagnetic field variability could be obtained. Nevertheless, the excursion is documented in inclination and declination, thus as a full reversal, and is associated with a deep low in relative paleointensity. At site PC04, the Iceland Basin excursion (∼195 ka) was also recorded as a short almost reversed phase, followed by an apparently long phase of non‐dipolar directions, all associated with a deep low in paleointensity. This is interpreted as the result of variable sedimentation rates not resolved by the preliminary age model of the core. A directional anomaly coinciding with a deep low in paleointensity at around 220 ka is interpreted as the Pringle Falls excursion. Besides these findings, no evidence for further excursions could be found in the studied cores.

Accurate reconstructions of export production in the Subantarctic Zone of the Southern Ocean are crucial for understanding the carbon cycle during Earth's past. However, due to the strong bottom water circulation of the Antarctic Circumpolar Current, sediment redistribution complicates age‐model‐derived bulk mass accumulation rates (BMAR). Here, we assess export production and its drivers over the past ∼1.4 Myr near the Drake Passage entrance using BMAR of biogenic barium, organic carbon, biogenic opal, calcium carbonate, and iron from sediment core PS97/093‐2, all of which are corrected for lateral sediment redistribution (corr‐BMAR). To quantify this correction, we explore the relationship between sortable silt as a bottom current strength proxy and ²³⁰Th‐derived focusing factors as indicators of lateral redistribution of sediments, respectively. Our findings highlight peak Fe input prior and during glacials of the Mid‐Pleistocene Transition (MPT), likely driven by enhanced Patagonian weathering. The carbonate record indicates increased deep‐ocean corrosivity after around 1 Ma ago and displays a shift in the accumulation pattern post‐MPT, with only isolated peaks in some peak interglacials. The high carbonate values during MIS 11 likely relate to Gephyrocapsa coccolithophore propagation, preceded and followed by prolonged carbonate dissolution periods, possibly linked to the Mid‐Brunhes Event. After the MPT, productivity proxies respond to glacial and interglacial intensity, with maxima found during MIS 16, MIS 11, MIS 5, and the Holocene, while minima occur during MIS 15–12. Our findings offer insights into long‐term productivity dynamics and their relationship to important climatic events over the past 1.4 Myr.

The Drake Passage is characterized by strong ocean currents barely allowing the deposition of fine grained sediments. Only in smaller basins protected from these currents sediments are able to settle more or less continuously. Two sediment cores from within the Drake Passage were subjected to magnetostratigraphic analyses. In one core inclinations are too steep while they are too shallow in the other one. Tentatively, directions of both cores were slightly tilted so that the maximum of the inclination distribution aligns with the direction of a geocentric axial dipole. Inclination variations then correlate fairly well, while declinations still show only little congruence. This is interpreted as the result of locally varying bottom currents partly biasing the remanence acquisition processes. Nevertheless, due to the high latitude of the coring site at 58°S, the field vector is mostly dominated by inclination and intensity variations. Directional variations during the documented Mono Lake (34.5 ka) and Laschamps (41.0 ka) geomagnetic excursions are only slightly changed by the applied tilt‐correction and afterward correlate very well from core to core. The Mono Lake excursion is characterized by shallow inclinations only, indicating a non‐axial dipolar field geometry. The field vector during the Laschamps excursion reaches a fully reversed direction. Both excursions are associated with clear minima in paleointensity. During the Laschamps excursion even a slight field recovery can be observed during the reversed phase of the field vector. Both excursions in Drake Passage sediments are terminated fairly abruptly followed by a more or less steep increase in paleointensity.

Dormancy is a wide-spread key life history trait observed across the tree of life. Many plankton species form dormant cells stages that accumulate in aquatic sediments and under anoxic conditions, form chronological records of past species and population dynamics under changing environmental conditions. Here we report on the germination of a microscopic alga, the abundant marine diatom Skeletonema marinoi that had remained dormant for up to 6871 ± 140 years in anoxic sediments of the Baltic Sea and resumed growth when exposed to oxygen and light. Resurrected diatom strains, representing cohorts from 6 different time points of the past 6871 ± 140 years, are genetically differentiated and fundamental physiological functions such as growth and photosynthesis had remained stable through time, despite distinct environmental dynamics. Showing that resurrection and full functional recovery, in comparison to 3 ± 2 years of dormancy, is possible after millennial resting, we emphasize the relevance of dormancy and living sediment archives. For the future, sediment archives, together with the resurrection approach would offer a powerful tool to trace adaptive traits over millennia under distinct climatic conditions and elucidate the underlying mechanisms.

Present-day sediment influx to the northern Red Sea is dominated by aeolian dust because of its position between the large deserts of northern Africa, the Arabian Peninsula and the Levant, and the absence of discharge from perennial rivers. However, sediment cores retrieved from the northern Red Sea reveal strong temporal variability of dust influx to the basin on glacial-interglacial timescales and several shorter-term strong episodes of fluvial input. We report new palaeoclimate and sediment provenance records for the last ca. 220 kyr from marine sediment core KL23, retrieved from the northern part of this basin. Our data suggest that the Nile delta became a major dust source during glacial conditions, in response to the glacioeustatic sea-level fall and associated subaerial exposure of volcanic-rich debris originally transported down the river Nile from the Ethiopian Highlands. Windblown dust from this delta source is characterized by high smectite concentrations and Ti contents. It is transported to the northern Red Sea on prevailing NNW winds. Our data also suggest a contribution of kaolinite-rich windblown dust from Egypt, Sinai and the Levant to KL23 with the same winds. The activity of this source is hydrologically controlled, with minima in kaolinite concentrations documenting phases of increased humidity, probably due to enhanced Mediterranean cyclogenesis and a southward expansion of the Mediterranean winter rains. Short-term reactivations of wadi systems during fluvial episodes are identified by fine grain sizes, high chlorite concentrations and high εNd. These episodes correlate with phases of reduced aeolian influx to the northern Red Sea and coincide with African Humid Periods, both in timing and relative intensity. This result implies that the Mediterranean climate system and the African monsoon are closely coupled.