Michael F. Thomas

Landscape sensitivity in time and space — an introduction

Abstract Landscape sensitivity may be discussed in terms of the response of landscape systems to perturbation on different time and spatial scales. Unstable systems behave chaotically but may show self organised criticality, while stable systems resist change until threshold values of system parameters are exceeded. Spatial sensitivity is expressed in different rates of change, between landscape components or elements. This leads to divergence between landscape elements, and the inheritance of palaeoforms in present-day landscape mosaics. Temporal sensitivity reflects the magnitude and frequency of individual events nested within patterns of longer term environmental changes occurring on different timescales. The resulting landscape complexity reflects the spatio-temporal sensitivity of earth surface systems over ten orders of scale magnitude. The connectivity within landscapes ensures that site instabilities can be propagated within multievent feedback systems. Landscapes record their own histories in sediments and soils, but interpretation of event stratigraphy may not be straightforward, while soil profiles can absorb individual events without erosion. Although we are increasingly able to model the present, environmental management is dominantly about conserving inherited properties of landscapes: forests, soils, floodplains, coastlines. Landscape sensitivity for landscape management must, therefore, address not only active, largely nonlinear, environmental systems, but also the mosaics and palimpsests that are the inheritance from past environments.

Grus weathering mantles—problems of interpretation

Abstract Grus is an ill-defined product of deep weathering of coarse-grained rocks whose relationships to other weathering changes remain unclear. This paper attempts to address this issue by reviewing a number of examples of coarse saprolites from a variety of climatic and topographic settings. Grus is the category of weathering mantle that possesses the following characteristics: sand+gravel 75–100%; silt+clay

Geomorphic response to rapid climatic and hydrologic change during the late pleistocene and early holocene in the humid and sub-humid tropics

Abstract The data base concerning late Quaternary environmental change in the humid tropics is dependent on records from scattered sites; the intensity and duration of wet-dry oscillations remain speculative, and the responses of hillslopes and river systems are based on over-simplified models. However, available evidence indicates that prolonged aridity affected all but a few favoured core areas of equatorial climate after 20,000 BP, lasting 5–7 ka. Dry conditions at the LGM were marked by semi-arid landforms and reduced steam activity. Large palaeofloods occurred after 13,000 BE but dry conditions returned after 11,000 BP, before the early Holocene pluvial led to abundant sedimentation lasting nearly 2 ka after 9500 BP, in Africa, Amazonia and Australasia. Erosion (channel cutting) and flood deposition occurred at the Pleistocene-Holocene transition, followed by multiple shifts from lateral to vertical accretion (fill-cut episodes) during the early Holocene, while thick overbank deposits formed during the later Holocene. Re-establishment of the lowland rainforests was delayed until after 9000 BP in Africa, Australia and Brazil and several wet-dry oscillations followed in the mid Holocene period. Hillslope activity at the LGM was marked by local sediment transfers and fan formation; at the termination sediment fluxes increased rapidly, but mass movement probably peaked after 10,000 BP Sediment delivery to stream channels was not immediate and large sediment stores remain on the landscape today, many in areas of potential sensitivity of erosion.

Landscape sensitivity to rapid environmental change¿a Quaternary perspective with examples from tropical areas

Abstract A millennial scale of ‘rapid’ change during the Quaternary is recorded in GRIP and GISP2 ice-core records, and is also found in tropical oceans. Rapid warming episodes followed by gradual cooling, associated with sub-Milankovich cycles implies an asymmetry in the behaviour of climate that is likely to be reflected in landscape responses. Slope failures, floods, colluvial/alluvial sedimentation may reflect short-term changes in the record, but reorganisation of slope and fluvial systems involves significant time lags or delays, often on a millennial scale, and requiring changes to the vegetation cover. While some records of sedimentation indicate major landscape instability during last 20 ky, or in the early Holocene, others indicate pulses of activity throughout the Last Glacial Cycle (LGC). These differences may reflect regional patterns of climate change, but also illustrate the importance of landscape sensitivity to our understanding of the impacts of rapid environmental change. Methodological problems arise in attempting correlations between site-specific records of sedimentation and integrated signals of hydrologic change found in deltaic and off-shore fans.

Alluvial fans, landslides and Late Quaternary climatic change in the wet tropics of northeast Queensland

Extensive alluvial‐fan and debris‐flow deposits occur along the base of the escarpment of the east Australian highlands in the wet tropics of northeast Queensland. Luminescence and radiocarbon dating show that these deposits accumulated between 27 ka and 14 ka, which was the driest phase of climate during the last full glacial cycle. Climatic desiccation and reduced plant cover, along with a continuation of discrete high‐magnitude rainfall events, were the principle causes of this phase of enhanced slope instability. Landslide activity and alluvial‐fan development have continued throughout the Holocene, but probably to a lesser extent and magnitude because of the amelioration of climate and the re‐establishment of forests throughout the region.

Late Quaternary stream sedimentation in the humid tropics: a review with new data from NE Queensland, Australia

There is now a wide agreement that temperature depression in the humid tropics during the Last Glacial Maximum (LGM) was at least 5°C. Most estimates of precipitation reduction at the LGM range from 25–30% to 50–65%, based on proxy data, but the recent CCM1 model envisages only around 12%. Dates obtained from river sediments indicate major changes to fluvial activity in the late Quaternary. Isotope Zone 3 sediments (58–28 ka BP) are widespread and possibly indicate cooler conditions. Post-28 ka BP, and certainly post-21 ka BP, river regimes altered radically towards fan building, braiding or major reduction in all activity. This paper reports on fan formation in NE Queensland between 26 and 14 ka BP and reviews evidence for comparable changes in humid tropical areas of S America, W Africa and SE Asia, including records of Holocene sedimentation. Within a global rhythm of major changes to river regimes in the humid tropics during the late Quaternary, it is now possible to detect regional variations in stream response to climatic change. At the LGM, reductions in stream power may have led to fan formation in NE Queensland, while vegetation changes may have contributed to increased sediment loads and braiding in some forest marginal areas. But, in W Africa, greater aridity may have been responsible for enfeebled streams leaving few records. Channel cutting, then deposition of coarse sediment in braided rivers marked the transition to the early Holocene in W Africa, and fans became entrenched in NE Queensland. This regime persisted until forest recovery was complete by 9.5–8.5 ka BP, when widespread overbank deposition occurred and a change towards meandering channels took place widely across the humid tropical zone, followed by several cut-and-fill episodes in the middle and late Holocene.

A geomorphological approach to geodiversity - its applications to geoconservation and geotourism

A geomorphological approach to geodiversity - its applications to geoconservation and geotourism Geodiversity is becoming widely considered alongside biodiversity by conservation agencies and has importance for geotourism. Geomorphology has a central role in understanding geodiversity, particularly at regional and local scales. By focusing on the processes that interact at the earths surface and how they respond to external forcing, geomorphology analyses both landscape evolution and real-time changes over different timescales. Diversity reflects the complexity of process systems and history. Connectivity and sensitivity amongst landscape elements are highly varied over space and time, leading to divergence and increasing diversity over time. By using these principles within constrained chronologies of landscape change, studies of geodiversity can become a valuable tool in ecosystem management and the delivery of ecosystem services, including sustainable geotourism.

Models for landform development on passive margins. Some implications for relief development in glaciated areas

Models for pre-glacial relief along passive continental margins must reflect the style and rate of uplift, and the rates of weathering and erosion, during at least the last 100-150 Ma. Passive margins have varied elevation, patterns of rifting, uplift and flexuring. A distinction is recognised between low-lying and steeply rising margins, the latter associated with major crustal upwarps. Continuous flexuring of the crust will maintain disequilibrium in weathering systems and known or possible rates of weathering and erosion appear compatible with weathering penetration during uplift. Under hot-humid conditions a rate of weathering of 50-100 mm ka −1 is possible and could produce 15-30 m of saprolite in 1 Ma (with 70 % surface loss), while long-term denudation rates seldom exceed 40 mm ka −1 . Deep, sandy and corestone profiles in zones of marginal dissection indicate effectiveness of weathering during uplift. Examples from southern Africa, southern Australia, South America and western Africa illustrate 4 morpho-tectonic settings: low elevation with minimum tilt, low domal uplift with coastal tilt, high elevation steeply arched, and high elevation faulted margin with back tilting. These examples are compared with higher latitude marginal landscapes in southeastern USA, Fennoscandia and Scotland. It is concluded that pre-glacial survival implies more than small relict features and includes: exhumed surfaces from beneath sedimentary covers; basins and troughs following zones of deeper weathering, and local planations, now warped and elevated to varying heights, above which inselbergs are locally common. While relict saprolites are found beneath duricrusts in the tropics, and strata-bound in high latitudes, most saprolite is better understood as having a dynamic history throughout the Neogene, including the Quaternary, whether in continuously warm or periodically cold climates

Late Quaternary sediment fluxes from tropical watersheds

Abstract Inherited saprolite stores and continued weathering in Quaternary time juxtapose abundant clay and fresh rock in tropical landscapes. This influences sediment fluxes and affects the interpretation of sediment sequences derived from tropical watersheds. Detrital kaolinites derive from inherited saprolite sources as well as from soil clays and appear in delta and ocean sediments. These sediments appear to correspond with sub-Milankovitch, millennial-scale cycles of climate change, but may also record century-scale episodes of rapid warming (Dansgaard–Oeschger events). Destabilisation of sediment sources and increased sediment fluxes in the Late Quaternary followed millennia of climatic deterioration (cooling/aridity) and vegetation change and led to altered patterns of sedimentation during the Last Glacial Maximum (LGM). Sediment yield from slopes increased 10× around the LGM, when rainfall was reduced by 30–60% and led to fan building and braided channels. Rainfall increased 40–80% from the LGM to the Early Holocene maximum and this led to channel cutting and major sediment fluxes to delta and ocean sinks. Vegetation recovery lagged the rapid warming by several millennia and was interrupted by (Younger Dryas) YD aridity, influencing slope and stream behaviour. Holocene sedimentation has been by both vertical and lateral accretion, increasing floodplain sediment stores.

The timing of alluvial sedimentation and floodplain formation in the lowland humid tropics of Ghana, Sierra Leone and western Kalimantan (Indonesian Borneo)

Abstract Temporal patterns in floodplain genesis and alluvial sedimentation in lowlands tropical rain forest zones of Ghana, Sierra Leone and western Kalimantan (Indonesian Borneo) based upon 14 C age determinations are described. Alluvial low terraces or buried sediments in West Africa yielded ages of 36-21 ka. In west Kalimantan a widespread episode of alluviation has yielded dates of 54-51 ka. The 20-13 ka interval was characterised by channel incision with valley floor erosion and neither region records sedimentation. Holocene alluvial sedimentation and floodplain construction in West Africa occurred during two temporal intervals: 10-7 ka and 4 ka to present and in western Kalimantan in response to early Holocene sea level rise followed by late Holocene regression and coastal outgrowth. The clustering of 14 C dates closely corresponds to regional lake level fluctuations and vegetational changes and to global indications of climatic change. We propose that periods of more frequent episodes of accelerated floodplain erosion and reconstitution, channel morpho-sedimentary activity and alluvial accumulation (1) are responses to interstadial and interglacial periods of higher precipitation following intervening periods of cooler and drier conditions; and (2) may be synchronous during the last 60 ka throughout the African and Asian inner humid lowland tropics.