Abstract Despite creating one of Earth's most iconic landforms, and having been studied since antiquity, river meandering is currently explained by several alternative models that remain debated. This is partly because observers have deconstructed meanders to a set of properties thought to be fundamental or significant for meandering, but each of these underpinning properties is itself complex and in turn requires analysis of its causes and mechanisms. This philosophical contribution aims to provide a perspective on the state of the science that makes a rigorous interrogation into what ‘causation’ actually means, and explores what a systems perspective can elucidate about meandering phenomena. We approach the problem through a discussion of the variety of ways in which previous researchers have tackled the problem of meandering, from a channel and floodplain perspective, on Earth and Mars, in the field and laboratory, and at the present day through to the ancient stratigraphic record. It is evident that river meandering has numerous causal pathways rather than a single set of necessary and sufficient conditions. A systemic view on the evolution of plant species shows that their feedbacks on river pattern likewise have several pathways. Employing the conceptual apparatus of the philosophy of science will move us towards a more consensual synthesis of river meandering.

Abstract A series of vegetation changes take place in tropical ecosystems during the Pennsylvanian Subperiod. The most notable change, recognizable from palynology and plant macrofossils at the Middle and Late Pennsylvanian boundary in the Illinois Basin, is the extirpation, or local extinction, of certain lineages of arborescent lycopsids, followed by their replacement by stem group marattialean tree ferns. The leading hypothesis suggests a significant change in precipitation regime as the cause. To test this hypothesis, we examine the vascular anatomy and physiology of key lineages of Pennsylvanian plants: the sphenopsids, tree ferns, cordaitaleans, medullosans, lycophytes and extrabasinal stem group coniferophytes. Using scanning electron and light microscopy of fossilized anatomy, we provide new data on these plants’ vascular systems, quantifying their physiological capacity and drought resistance. We find that three Pennsylvanian plant lineages – the medullosans, arborescent lycopsids and Sphenophyllum – contain high hydraulic conductivity but are vulnerable to drought-induced damage, whereas others are resistant, including stem group tree ferns and coniferophytes. Relative abundance changes among these plants were likely driven by drought, and differences in water use efficiency would have amplified drought events as plant communities changed. The interaction of physiological selectivity and positive feedback between aridity and drought tolerance likely played a significant role in Late Paleozoic floral changes.

ABSTRACT The late early to middle Miocene period (18–12.7 Ma) was marked by profound environmental change, as Earth entered into the warmest climate phase of the Neogene (Miocene climate optimum) and then transitioned to a much colder mode with development of permanent ice sheets on Antarctica. Integration of high-resolution benthic foraminiferal isotope records in well-preserved sedimentary successions from the Pacific, Southern, and Indian Oceans provides a long-term perspective with which to assess relationships among climate change, ocean circulation, and carbon cycle dynamics during these successive climate reversals. Fundamentally different modes of ocean circulation and carbon cycling prevailed on an almost ice-free Earth during the Miocene climate optimum (ca. 16.9–14.7 Ma). Comparison of δ 13 C profiles revealed a marked decrease in ocean stratification and in the strength of the meridional overturning circulation during the Miocene climate optimum. We speculate that labile polar ice sheets, weaker Southern Hemisphere westerlies, higher sea level, and more acidic, oxygen-depleted oceans promoted shelf-basin partitioning of carbonate deposition and a weaker meridional overturning circulation, reducing the sequestration efficiency of the biological pump. X-ray fluorescence scanning data additionally revealed that 100 k.y. eccentricity-paced transient hyperthermal events coincided with intense episodes of deep-water acidification and deoxygenation. The in-phase coherence of δ 18 O and δ 13 C at the eccentricity band further suggests that orbitally paced processes such as remineralization of organic carbon from the deep-ocean dissolved organic carbon pool and/or weathering-induced carbon and nutrient fluxes from tropical monsoonal regions to the ocean contributed to the high amplitude variability of the marine carbon cycle. Stepwise global cooling and ice-sheet expansion during the middle Miocene climate transition (ca. 14.7–13.8 Ma) were associated with dampening of astronomically driven climate cycles and progressive steepening of the δ 13 C gradient between intermediate and deep waters, indicating intensification and vertical expansion of ocean meridional overturning circulation following the end of the Miocene climate optimum. Together, these results underline the crucial role of the marine carbon cycle and low-latitude processes in driving climate dynamics on an almost ice-free Earth.