Ann Holbourn

Impacts of orbital forcing and atmospheric carbon dioxide on Miocene ice-sheet expansion.

The processes causing the middle Miocene global cooling, which marked the Earths final transition into an ‘icehouse’ climate about 13.9 million years ago (Myr ago), remain enigmatic. Tectonically driven circulation changes and variations in atmospheric carbon dioxide levels have been suggested as driving mechanisms, but the lack of adequately preserved sedimentary successions has made rigorous testing of these hypotheses difficult. Here we present high-resolution climate proxy records, covering the period from 14.7 to 12.7 million years ago, from two complete sediment cores from the northwest and southeast subtropical Pacific Ocean. Using new chronologies through the correlation to the latest orbital model, we find relatively constant, low summer insolation over Antarctica coincident with declining atmospheric carbon dioxide levels at the time of Antarctic ice-sheet expansion and global cooling, suggesting a causal link. We surmise that the thermal isolation of Antarctica played a role in providing sustained long-term climatic boundary conditions propitious for ice-sheet formation. Our data document that Antarctic glaciation was rapid, taking place within two obliquity cycles, and coincided with a striking transition from obliquity to eccentricity as the drivers of climatic change.

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Neogene History of the Indonesian Throughflow

The Indonesian Throughflow acts as a major switchboard in the global thermohaline circulation, and its variability is strongly related to tropical climate dynamics on shorter and longer timescales. During the Holocene and Pleistocene, fluctuating sea surface temperature and salinity patterns in the Western Pacific Warm Water Pool and Indonesian Seas and variations in East Asian monsoon strength mainly controlled the intensity and hydrological characteristics of the throughflow. Additionally, glacial/deglacial sea-level change strongly influenced throughflow volume in shallow sections of many passages (i.e. the southern part of the Timor passage on the NW Australian shallow shelf) thus altering the related heat transfer between oceans. The tectonic history of the Indonesian Gateway ultimately controlled the long-term evolution of the throughflow. During the Pliocene, changes in the position and geometry of the inflow passages (Mindanao Passage to the North and Halmahera Passage to the south) in relation to the tropical Pacific front significantly modified the climatic role of the tropical Indian and Pacific Oceans, resulting in reduced atmospheric heat transport from the tropics to high latitudes. However, the precise timing of major restriction in the surface and thermocline water flow is difficult to ascertain. The early evolution of the Indonesian Gateway was characterized by tectonic restriction of the deep water pathway between the Pacific and Indian Oceans at approximately 25 Ma. By the early Miocene, the Indonesian Gateway was already closed as a deep water pathway between the Pacific and Indian Oceans.

Transient global cooling at the onset of early Aptian oceanic anoxic event (OAE) 1a

The onset of the early Aptian oceanic anoxic event (OAE) 1a (ca. 120 Ma) coincided with a major perturbation of the carbon cycle, which is reflected in the sedimentary carbon isotope record. Triggering mechanisms, duration, and climatic repercussions of this episode of accelerated organic matter burial remain poorly constrained. Here, we present millennial-scale bulk rock carbon and oxygen isotope data from a marly subtropical intrashelf basin (La Bedoule, southeast France) with unusually high sedimentation rates, which track the onset of OAE1a in unprecedented resolution. Our record reveals that the negative, low-amplitude delta(13)C excursion preceding OAE1a lasted >100 k.y., implying that enhanced volcanic CO(2) emission and/or pulsed methane dissociation over a prolonged time span were instrumental in triggering OAE1a. The main positive carbon isotope shift at the onset of OAE1a, previously regarded as continuous, occurred stepwise over an extended period of >300 k.y. Transient climate cooling during the initial delta(13)C increase probably reflects ephemeral high-latitude glaciation, triggered by changes in radiative forcing and drawdown of atmospheric CO(2).

Middle Miocene climate cooling linked to intensification of eastern equatorial Pacific upwelling

During the Middle Miocene, Earths climate transitioned from a relatively warm phase (Miocene climatic optimum) to a colder mode with reestablishment of permanent ice sheets on Antarctica, thus marking a fundamental step in Cenozoic cooling. Carbon sequestration and atmospheric CO2 drawdown through increased terrestrial and/or marine productivity have been proposed as the main drivers of this fundamental transition. We integrate high-resolution (1-3 k.y.) benthic stable isotope data with X-ray fluorescence scanner-derived biogenic silica and carbonate accumulation estimates in an exceptionally well preserved sedimentary archive, recovered at Integrated Ocean Drilling Program Site U1338, to reconstruct eastern equatorial Pacific productivity variations and to investigate temporal links between high- and low-latitude climate change over the interval 16-13 Ma. Our records show that the climatic optimum (16.8-14.7 Ma) was characterized by high-amplitude climate variations, marked by intense perturbations of the carbon cycle. Episodes of peak warmth at (Southern Hemisphere) insolation maxima coincided with transient shoaling of the carbonate compensation depth and enhanced carbonate dissolution in the deep ocean. A switch to obliquity-paced climate variability after 14.7 Ma concurred with a general improvement in carbonate preservation and the onset of step-wise global cooling, culminating with extensive ice growth over Antarctica ca. 13.8 Ma. We find that two massive increases in opal accumulation ca. 14.0 and ca. 13.8 Ma occurred just before and during the final and most prominent cooling step, supporting the hypothesis that enhanced siliceous productivity in the eastern equatorial Pacific contributed to CO2 drawdown.

BENTHIC FORAMINIFERS FROM LOWER ALBIAN BLACK SHALES (SITE 1049, ODP LEG 171): EVIDENCE FOR A NON “UNIFORMITARIAN” RECORD

A comparison of benthic foraminifers from lower Albian black shales at Ocean Drilling Program Site 1049 with modern and Mesozoic high carbon flux/low oxygen assemblages indicate that no preferential test shape characterizes benthic foraminifers from dysaerobic environments, as almost the entire spectrum of test morphologies is represented within these assemblages. The lower Albian benthic foraminiferal assemblages from Site 1049 are the first black shale assemblages to be dominated by Fursenkoina viscida, Ellipsoidella cuneata, Pleurostomella reussi and Osangularia schloenbachi all members of the suborder Rotaliina. Thus, early Albian black shale assemblages differ markedly from Aptian and Jurassic assemblages dominated mainly by representatives of the suborders Textulariina and Lagenina. Members of the suborder Rotaliina continued to radiate profusely during the Late Cretaceous and Cenozoic, eventually giving rise to a large proportion of the modern fauna, including many of the high productivity/low oxygen taxa. There is no evidence for a progressive morphological adaptation to low oxygen with time.

Monitoring the recolonization of the Mt Pinatubo 1991 ash layer by benthic foraminifera

Benthic foraminifera from the South China Sea were studied to assess mass mortality and to monitor the composition and recovery of the benthic communities following the 1991 Mt Pinatubo ashfall. Surface distribution data from monitoring stations in the eastern South China Sea that were occupied during four cruises between spring 1994 and summer 1998 display the following trends in recolonization patterns: (1) Suspension feeding epifaunal benthic foraminifera (i.e. Cibicidoides wuellerstorfi, Saccorhiza ramosa) and large xenophyophores (i.e. Syringammina (?)fragilissima) were absent in spring 1994 and only rare individuals were observed in June 1996, but in larger numbers in December 1996 and in summer 1998. Then, they were important recolonizers of the ash layer. (2) Diversity and population densities have changed significantly since 1994. Following an abundance maximum in winter 1996, the numbers of living individuals in summer 1998 decreased again and the deep sea benthic foraminiferal community started to return to a normal ecological structuring. However, infaunal foraminifera were still strongly dominated by several species of the genus Reophax. We interpret the changing abundance and diversity pattern during the recolonization process in two ways: (1) the markedly increasing activity of burrowing macrofauna observed since 1998 opened new ecological niches for infaunal benthic foraminifera but also intensified predator pressure; (2) competitive interactions within the recolonizing fauna began to play a major role. Opportunistic pioneer species, characterized by rapid reproduction rates and the capability to colonize disturbed environments, were outcompeted by non-opportunistic species

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Bryozoan reef mounds are common features in the geological record, occurring within mid-ramp, slope paleoenvironments, especially in Paleozoic carbonate successions, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (Ocean Drilling Program Leg 182) has confirmed the existence of shallow subsurface bryozoan reef mounds in modern water depths of 200–350 m. These structures have as much as 65 m of synoptic relief, and occur both as single mounds and as mound complexes. They are unlithified, have a floatstone texture, and are rich in delicate branching, encrusting and/or nodular-arborescent, flat-robust branching, fenestrate, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules, and calcareous nannofossils. The 14C accelerator mass spectrometry dates of 26.6–35.1 ka indicate that the most recent mounds, the tops of which are 7–10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgressive sea-level rise. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, and reduced upwelling and lowered trophic resources during highstands. Large specimens of benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition, and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.

Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)

We investigated the sequence architecture of two expanded Cenomanian successions along a depth transect in the Tarfaya Basin (SW Morocco) and correlated these successions to published records from northwest Europe and India. Changes in terrigenous material, carbonate and organic carbon content, carbonate microfacies and foraminiferal biofacies, as well as nondepositional and erosional surfaces were used to define depositional sequences and systems tracts. We identified two main transgressive cycles in the lower and middle-upper Cenomanian separated by a major regression at the early-middle Cenomanian transition (sequence boundary Ce 3). This regressive interval is characterized by lagoonal low-stand deposits indicating an overall sealevel fall of more than 30 m. Superimposed on the two main transgressive cycles, there are 11 third-order depositional sequences that correlate to globally recognized sealevel fluctuations and appear to be paced by long eccentricity variations (400 Ka period). Positive carbon isotope excursions in the middle Cenomanian (96.0 Ma) and latest Cenomanian (94.0 Ma) following sealevel lowstands together with planktonic foraminiferal and ammonite datums provide a robust framework for stratigraphic correlation. We suggest that the onset of these excursions was triggered by eccentricity minima during periods of low variability in obliquity (nodes), which probably coincided with glacioeustatic lowstands.

Changes in Northeast Atlantic temperature and carbon flux during the Cenomanian/Turonian paleoceanographic event: the Goban Spur stable isotope record

Stable isotopes of bulk sediment and well preserved tests of planktonic and benthic foraminifera from midlatitude NE Atlantic DSDP Site 551 (Goban Spur) provide the first estimates of carbon isotope gradients within the water column at a lower bathyal site during the Cenomanian/Turonian boundary interval (CTBI). The CTBI carbon isotope excursion is prominent (up to 2parts per thousand shift in delta(13)C) in the bulk (coccolith) signal, but less pronounced (approximately 0.5 parts per thousand shift in delta(13)C) in planktonic and benthic foraminifera. This difference indicates a very steep (13)C gradient in the upper water column and a very efficient biological pump during the CTBI carbon isotope excursion. We suggest significantly increased seasonal primary production in the uppermost water column with an enhanced shallow water chlorophyll maximum as a cause for this steep carbon isotope gradient. Deep-water and surface-water temperature changes during the CTBI are estimated using benthic and planktonic foraminiferal oxygen isotopes. Warm deep-water masses (13-16degreesC) and a low temperature gradient within the water column prevailed in the late Cenomanian. Additional warming (approximately 2degreesC for both surface and deep water) occurred in the latest Cenomanian prior to CTBI black shale deposition. This pattern of CTBI black shale deposition during a temperature maximum is also evident at two low latitude locations (ODP Site 1050, Blake Nose and Tarfaya, southern Morocco)