Steven C. Clemens

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

Nonstationary Phase of the Plio-Pleistocene Asian Monsoon

Paleoclimate records indicate that the strength of the Asian summer monsoon is sensitive to orbital forcing at the obliquity and precession periods (41,000 and 23,000 years, respectively) and the extent of Northern Hemisphere glaciation. Over the past 2.6 million years, the timing (phase) of strong monsoons has changed by ∼83 degrees in the precession and ∼124 degrees in the obliquity bands relative to the phase of maximum global ice volume (inferred from the marine oxygen isotope record). These results suggest that one or both of these systems is nonstationary relative to orbital forcing.

A 350,000 year summer-monsoon multi-proxy stack from the Owen Ridge, Northern Arabian Sea

Five summer-monsoon proxies from the Northern Arabian Sea are combined using stacking and principal components analysis (PCA) to create two very similar multi-proxy records of summer-monsoon variability. The five individual proxies all respond to monsoon variability but are largely independent in terms of the processes that complicate their interpretation as summer-monsoon indicators (e.g. preservation, dissolution, diagenesis, sediment reworking). As such, stacking and PCA average out non-monsoon variance, yielding a more pure monsoon signal. These stacked and PCA records (hereafter summer-monsoon stack and summer-monsoon factor) allow evaluation of relative monsoon strength through time as well as the relative concentration of variance within orbital bands; these two parameters are less reliable when estimated from individual proxy records. In fact, the summer-monsoon factor (SMF) accounts for only 33% of the total variance in the five records, suggesting that relative amplitude variations in each individual proxy time series are influenced by non-monsoon processes. The summer-monsoon stack (SMS) and SMF are spectrally very similar, dominated by variance in the 41-k.y. (obliquity) and 23-k.y. (precession) bands; there is very little variance at the 100-k.y. (eccentricity) band associated with large-scale changes in global ice-volume. Indeed, equally strong monsoons occur in both glacial and interglacial intervals. Within the 23-k.y. precession cycle, monsoon maxima fall at −121° relative to precession minima (June 21 perihelion, maximum Northern Hemisphere (NH) summer insolation). This phase falls midway between δ18O minima (−78°) and December 21 perihelion (−180°) indicating that two mechanisms exert equal influence in determining the timing of strong summer monsoons within the precession band: (1) sensible heating of the Asian Plateau which is maximized at times of ice-volume minima (−78°), and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of December 21 perihelion (−180°). The seasonal cycle at December 21 perihelion is characterized by warm Southern Hemisphere (SH) summers followed by cold SH winters, a combination that preconditions the ocean to export latent heat during the boreal summer-monsoon season. Summer-monsoon winds transport this latent heat into Asia where it is released during precipitation, enhancing the Asian monsoon low. Within the 41-k.y. obliquity cycle, monsoon maxima are in phase with obliquity maxima. This indicates that two mechanisms, quite similar to those in the precession band, influence the timing of strong summer-monsoons in the obliquity band: (1) sensible heating of the Asian Plateau but with no ice-volume delay, and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of obliquity maxima. Again, the seasonal cycle at obliquity maximum is characterized by warm SH summers followed by cold SH winters, ideal for maximizing latent heat export during the boreal summer monsoon.

Glacial-Interglacial Indian Summer Monsoon Dynamics

Indian summer monsoon changes during the Pleistocene were influenced by dynamic effects originating in both hemispheres. The modern Indian summer monsoon (ISM) is characterized by exceptionally strong interhemispheric transport, indicating the importance of both Northern and Southern Hemisphere processes driving monsoon variability. Here, we present a high-resolution continental record from southwestern China that demonstrates the importance of interhemispheric forcing in driving ISM variability at the glacial-interglacial time scale as well. Interglacial ISM maxima are dominated by an enhanced Indian low associated with global ice volume minima. In contrast, the glacial ISM reaches a minimum, and actually begins to increase, before global ice volume reaches a maximum. We attribute this early strengthening to an increased cross-equatorial pressure gradient derived from Southern Hemisphere high-latitude cooling. This mechanism explains much of the nonorbital scale variance in the Pleistocene ISM record.

Contrasting the Indian and East Asian monsoons: implications on geologic timescales

The surface winds over the Arabian Sea and South China Sea are meaningful indicators for the strength of the Indian monsoon and East Asian monsoon, respectively. Paleo-monsoon variability has been studied through analysis of sediment records from these two monsoon regions. To facilitate interpretation of these records, we focus on the impacts of ‘internal’ and ‘external’ forcing of the monsoon system by contrasting the annual cycle and interannual variability of two subsystems: the monsoon over the Indian sector (40^105‡E) and over the East Asian sector (105^ 160‡E). Differences in the annual cycle within these subsystems arise primarily from the different land^ocean configurations that determines atmospheric response to the solar forcing. Thus factors that drive intensities of the monsoonal annual cycle share common features with the external (geographic and orbital) forcing that controls paleomonsoon variability. We show that the differences in interannual variations between the two monsoon subsystems are primarily due to internal factors of the coupled atmosphere^ocean^land system, such as remote impacts of El Nin ‹ o/La Nin ‹ a and local monsoon^ocean interactions. The mechanisms that operate on interannual to interdecadal timescales may differ fundamentally from that on geologic/orbital timescales. The low-level flows over the East Asia and Australia are essentially established by geographic forcing. The amplification of the Australia summer monsoon during increased solar precession is likely caused by an enhanced East Asian winter monsoon, rather than following an enhanced Indian summer monsoon as on the interannual timescale. It is also found that El Nin ‹ o influences the low-level flow moderately over the Arabian Sea but to a greater extent over the South China Sea. As such, large changes in the Pacific thermal conditions may significantly alter the intensity of the East Asian monsoon but not the Indian monsoon.

Millennial and orbital variations of El Nino/Southern Oscillation and high-latitude climate in the last glacial period

The El Niño/Southern Oscillation (ENSO) phenomenon is believed to have operated continuously over the last glacial–interglacial cycle. ENSO variability has been suggested to be linked to millennial-scale oscillations in North Atlantic climate during that time, but the proposals disagree on whether increased frequency of El Niño events, the warm phase of ENSO, was linked to North Atlantic warm or cold periods. Here we present a high-resolution record of surface moisture, based on the degree of peat humification and the ratio of sedges to grass, from northern Queensland, Australia, covering the past 45,000 yr. We observe millennial-scale dry periods, indicating periods of frequent El Niño events (summer precipitation declines in El Niño years in northeastern Australia). We find that these dry periods are correlated to the Dansgaard–Oeschger events—millennial-scale warm events in the North Atlantic climate record—although no direct atmospheric connection from the North Atlantic to our site can be invoked. Additionally, we find climatic cycles at a semiprecessional timescale (∼11,900 yr). We suggest that climate variations in the tropical Pacific Ocean on millennial as well as orbital timescales, which determined precipitation in northeastern Australia, also exerted an influence on North Atlantic climate through atmospheric and oceanic teleconnections.

Improved chronostratigraphic reference curve of late Neogene seawater 87Sr/86Sr

We present a reference curve of seawater 87 Sr/ 86 Sr variation through the past 7 m.y. based on 455 samples of planktonic foraminifera from Ocean Drilling Program (ODP) Site 758 in the Indian Ocean. This single-site curve is superior to extant curves because of several features: (1) a continuous sedimentary section free of disturbances; (2) a well-determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale; (3) a high temporal resolution (sample interval averages 15 ka); (4) an improved analytical reproducibility; and (5) a uniformly high sample quality. The gross structure of the Site 758 curve is generally similar to that observed in previous work, but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution. At the 95% confidence level, uncertainties on age estimates range from ±0.60 m.y. in the lower Pleistocene and upper Miocene to ±2.03 m.y. in the middle Pliocene. Moreover, this detailed record identifies two periods of steplike increases in seawater 87 Sr/ 86 Sr (6.10 to 5.80 Ma and 1.46 to 1.13 Ma) that are characterized by rates of change of ∼150 × 10 −6 /m.y. These features, if verified at other sites, indicate abrupt changes in the Sr mass balance of the ocean and may be used as precise event markers in late Neogene marine sections.

Clay mineral assemblages in the northern South China Sea: implications for East Asian monsoon evolution over the past 2 million years

Abstract Clay mineral assemblages at ODP Site 1146 in the northern South China Sea are used to investigate sediment source and transport processes and to evaluate the evolution of the East Asian monsoon over the past 2 Myr. Clay minerals consist mainly of illite (22–43%) and smectite (12–48%), with associated chlorite (10–30%), kaolinite (2–18%), and random mixed-layer clays (5–22%). Hydrodynamic and mineralogical studies indicate that illite and chlorite sources include Taiwan and the Yangtze River, that smectite and mixed-layer clays originate predominantly from Luzon and Indonesia, and that kaolinite is primarily derived from the Pearl River. Mineral assemblages indicate strong glacial–interglacial cyclicity, with high illite, chlorite, and kaolinite content during glacials and high smectite and mixed-layer clay content during interglacials. During interglacials, summer enhanced monsoon (southwesterly) currents transport more smectite and mixed-layer clays to Site 1146 whereas during glacials, enhanced winter monsoon (northerly) currents transport more illite and chlorite from Taiwan and the Yangtze River. The ratio (smectite+mixed layers)/(illite+chlorite) was adopted as a proxy for East Asian monsoon variability. Higher ratios indicate strengthened summer-monsoon winds and weakened winter-monsoon winds during interglacials. In contrast, lower ratios indicate a strongly intensified winter monsoon and weakened summer monsoon during glacials. Spectral analysis indicates the mineral ratio was dominantly forced by monsoon variability prior to the development of large-scale glaciation at 1.2 Myr and by both monsoon variability and the effects of changing sea level in the interval 1.2 Myr to present.

Multiple expansions of C4 plant biomass in East Asia since 7 Ma coupled with strengthened monsoon circulation

The expansion of plants using the C4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the Cenozoic Era. A significant late Miocene expansion is well documented. However, the existence and cause of subsequent expansions are still not clear, owing in part to the lack of long, continuous climate-vegetation records. Here we present two high-resolution carbon isotope time series, covering the past 7 m.y., derived from eolian deposits on the Chinese Loess Plateau. The data indicate three intervals of significant C4 plant expansions within the semiarid monsoonal region of East Asia (ca. 2.9–2.7 Ma, 1.3–0.9 Ma, and 0.6 Ma–present). These expansions covary with strengthened East Asian summer monsoon circulation. We conclude that in East Asia, large-scale late Miocene C4/C3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation.

Large-scale hydrological change drove the late Miocene C4 plant expansion in the Himalayan foreland and Arabian Peninsula

Carbon isotope changes in paleosols from Siwalik, Pakistan, and marine sediments from the Bengal Fan indicate a major C 4 plant expansion in the Himalayan foreland during the late Miocene. However, the timing and mechanisms behind the C 4 plant expansion remain enigmatic. Here we present high-resolution (∼60 k.y.) biomarker and compound-specific isotope data spanning the past 11 m.y. from Ocean Drilling Program Site 722 in the Arabian Sea. An ∼5‰–6‰ increase in leaf wax δ 13 C values indicates a marked rise of C 4 plants from 10 to 5.5 Ma, with accelerated expansion from 7.9 to 5.5 Ma. A concurrent ∼50‰ rise in leaf wax δD values is attributed to a combined effect of changes in precipitation amount and evaporation, indicating that source regions for the plant waxes became progressively drier from 10 to 5.5 Ma. In contrast to earlier reports, our isotope records, biomarker abundances, alkenone U K′ 37 , and Globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval. Rather, our results suggest that large-scale hydrological changes drove the late Miocene expansion of C 4 plants in the Himalayan foreland and Arabian Peninsula.