Jérôme Kaiser

Archaea are single‐celled microorganisms that are abundant in marine environments and play a key role in the carbon and nitrogen cycles. Archaea can biosynthesize a wide variety of isoprenoid membrane lipids, including isoprenoid glycerol dibiphytanyl glycerol tetraethers (isoGDGTs). Experimental and empirical evidence demonstrates that the number of cyclopentane moieties in isoGDGTs is related to growth temperature, leading to the formulation and development of the TEX ₈₆ ocean temperature proxy. The TEX ₈₆ proxy has been widely applied to marine sediments spanning the last 190 million years and has revealed unique insights into the evolution of ocean temperatures over diverse timescales. Other indices have been developed using archaeal lipids to reconstruct ecological and biogeochemical processes that operate within the marine realm. However, several knowledge gaps (e.g., depth of export production, non‐temperature controls, application in non‐analogue climates) limit the application of archaeal lipid‐based proxies in modern and ancient marine environments. In this review, we critically assess the utility of archaeal lipids as tracers for (paleo) environmental change, with an emphasis on the TEX ₈₆ paleotemperature proxy.

The Baltic Sea has a long history of anthropogenic disturbance, that started earlier than in most other coastal oceans and marginal seas. Especially in regions where shallow depths and limited space constrain the area that is available for anthropogenic use, conflicts of interest arise from the rising demand of multiple socio-economic players such as offshore wind, nature conservation, shipping, coastal protection, fishing, military, tourism and many more. The intensive use over many centuries has left long-lasting and partly irreversible traces on the seafloor and the benthic ecosystem. We aim to make the traces of different seafloor modulating processes such as bottom trawling, ship anchoring, propeller wake erosion, seabed constructions or material dumping visible using marine geophysical data of different resolution and spatial coverage. From this data, we can derive the spatial distribution and intensity of anthropogenic disturbances in the Baltic Sea and subsequently evaluate the pressures that they exert in certain areas. This approach is exemplified for propeller wakes that are generated by commercial ships, and that are not yet included in cumulative impact assessments. The results outline how single processes can exert pressures on the entire vertical sea, from the ocean-atmosphere boundary down to the seafloor and below, with likely impacts on ecosystem functioning and marine biodiversity. For propeller wakes, the broad spectrum of direct consequences suggests that the challenges associated with this anthropogenic stressor can only be met and moderated through intensive interdisciplinary research.

Archaea are single-celled microorganisms that are abundant in marine environments and play a key role in the carbon and nitrogen cycles. Archaea can biosynthesize a wide variety of isoprenoid membrane lipids, including isoprenoid glycerol dibiphytanyl glycerol tetraethers (isoGDGTs). Experimental and empirical evidence demonstrates that the number of cyclopentane moieties in isoGDGTs is related to growth temperature, leading to the formulation and development of the TEX86 ocean temperature proxy. The TEX86 proxy has been widely applied to marine sediments spanning the last 190 million years and has revealed unique insights into the evolution of ocean temperatures over diverse timescales. Other indices have been developed using archaeal lipids to reconstruct ecological and biogeochemical processes that operate within the marine realm. However, several knowledge gaps limit the application of archaeal lipid-based proxies in modern and ancient marine environments. In this review, we critically assess the utility of archaeal lipids as tracers for (paleo)environmental change, with an emphasis on the TEX86 temperature proxy.

Centennial-timescale variability of the Southern Hemisphere Westerly wind belt (SHW) is poorly documented, mainly due to a paucity of long, high-resolution climate records from the Southern Hemisphere. Moreover, the role played by the Southern Annular Mode (SAM), El Niño Southern Oscillation (ENSO) and solar variability in controlling the SHW is not well understood. We present a 7000-year record of grain size end-member modelling and C/N from Lake Lanalhue at 38°S in central Chile as a reconstruction of variations in precipitation and the position of the northern margin of the SHW. Centennial-scale drier/wetter conditions in central Chile were coeval not only with drier/wetter and cooler/warmer conditions in southern Chile, but also with more/less extensive sea ice and cooler/warmer conditions in the Ross Sea, stronger/weaker winds in the Amundsen Sea, and equatorward/poleward shifted SHW in the southwestern Atlantic Ocean. Taken together this indicates centennial-scale variations resembling positive/negative phases of the Southern Annular Mode (SAM ⁺ /SAM ^– ). Drier/wetter conditions were also related to more/less El Niño conditions. Therefore, we suggest that SHW variability in central Chile at a centennial-timescale was related to the ENSO-SAM phasing, i.e. El Niño/SAM ⁺ during drier periods and La Niña/SAM ^– during wetter periods, and ultimately controlled by solar variability, with increased/decreased insolation forcing shifts towards a SAM ⁺ /SAM ^– -like state.

The Antarctic Circumpolar Current (ACC) is a major driver of global ocean circulation and climate. To better understand the interplay between long-term atmospheric and ocean variability in the Southern Ocean since the late Miocene, we present sea surface temperature (SST) and carbonate preservation records from the Subantarctic Eastern South Pacific (IODP Site U1543), along with an extended ACC strength record from Central South Pacific Site U1541. We focus on long-term eccentricity-scale variations showing decreased (increased) SST with enhanced (reduced) CaCO₃ preservation, and stronger (weaker) ACC strength, particularly during the Pliocene. These changes coincide with stronger (weaker) South Pacific SST gradients, possible northward (southward) migration of Southern Ocean fronts, strengthened (weakened) westerlies, and atmospheric CO₂ release. These patterns contrast with Pleistocene glacial-interglacial cycles. Reduced Pacific-Atlantic exchange through the Drake Passage may have weakened Atlantic Meridional Overturning Circulation during warming at Site U1543 across the intensification of Northern Hemisphere Glaciation. Simultaneous stronger ACC and higher CaCO₃ deposition in the high-latitude Pacific suggest a strengthened basin-wide Pacific overturning circulation during parts of the Pliocene.

Global warming is a major threat to marine biodiversity and ecosystem functioning, with consequences that are yet largely unknown. To frame these consequences, we need to understand how marine ecosystems respond to warming and related environmental changes. Ecosystem models have proven to be a valuable tool in this respect, but their projections vary considerably. A major limitation in current ecosystem models may be that they largely ignore evolutionary processes, which nonetheless can be relevant on the simulated timescales. In addition, ecosystem models are usually fit to contemporary data and used predictively afterwards, without further validation that they are equally applicable to past (and, by inference, future) scenarios. A promising approach to validate evolutionary ecosystem models is the use of biological archives such as natural sediments, which record long-term ecosystem changes. Since the ecosystem changes present in sediment records are affected by evolution, evolution needs to be represented in ecosystem models not only to realistically simulate the future, but also the sediment record itself. The sediment record, in turn, can provide the required constraints on long-term evolutionary changes, along with information on past environmental conditions, biodiversity, and relative abundances of taxa. Here, we present a framework to make use of such information to validate evolutionary ecosystem models and improve model projections of future ecosystem changes. Using the example of phytoplankton, key players in marine systems, we review the existing literature and discuss (I) which data can be derived from ancient sedimentary archives, (II) how we can integrate these data into evolutionary ecosystem models to improve their projections of climate-driven ecosystem changes, and (III) future perspectives and aspects that remain challenging.