Larry C. Peterson

On the Structure and Origin of Major Glaciation Cycles 1. Linear Responses to Milankovitch Forcing

Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind-driven and density-driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,000 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that the 23,000- and 41,000-year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean. Early in this progression, deep waters of the Southern Ocean appear to act as a carbon trap. To stimulate new observations and modeling efforts, we offer a process model that gives a synoptic view of climate at the four end-member states needed to describe the systems evolution, and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems. “Solutions to problems involving systems of such complexity are not born full grown like Athena from the head of Zeus. Rather they evolve slowly, in stages, each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step.” —Victor P. Starr

On the structure and origin of major glaciation cycles 2. The 100,000‐year cycle

Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000-year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000-year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000-year cycle requires a source of climatic inertia having a time constant (similar to 15,000 years) much larger than the other cycles (similar to 5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity-driven modulations in the 23,000-year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000-year oscillations. Whether a glacier or ice sheet influences the climate depends very much on the scale....The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence.

Influence of the intertropical convergence zone on the East Asian monsoon

The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.

Deglacial changes in ocean circulation from an extended radiocarbon calibration

Temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon-based age-estimates of biogenic material that do not agree with true calendar age. This problem is particularly severe beyond the limit of the high-resolution radiocarbon calibration based on tree-ring data, which stretches back only to, about 11.8 kyr before present (BP), near the termination of the Younger Dryas cold period. If a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation, extending the well-calibrated radiocarbon timescale back further in time is crucial. Several studies attempting such an extension, using uranium/thorium-dated corals and laminae counts in varved sediments, show conflicting results. Here we use radiocarbon data from varved sediments in the Cariaco basin, in the southern Caribbean Sea, to construct an accurate and continuous radiocarbon calibration for the period 9 to 14.5 kyr BP, nearly 3,000 years beyond the tree-ring-based calibration. A simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that North Atlantic Deep Water formation shut down during the Younger Dryas period, but was gradually replaced by an alternative mode of convection, possibly via the formation of North Atlantic Intermediate Water.

A high-resolution Late Quaternary upwelling record from the anoxic Cariaco Basin, Venezuela

Results are presented of a high-resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the Cariaco Basin in the southern Caribbean Sea. The Cariaco Basin is a small anoxic marine basin on the northern continental margin of Venezuela in an area today characterized by both seasonal trade wind-induced upwelling and pronounced dry and wet seasons. Our data indicate that large changes in the intensity of upwelling, and hence trade wind strength, occurred in this region during the last glacial-interglacial transition and throughout the Holocene. During the last glacial lowstand of sea level, the Cariaco Basin was effectively isolated from the open Caribbean along its northern margin by the then largely emergent Tortuga Bank. Oxic conditions existed in the deep Cariaco Basin at this time, and surface productivity was low. About 12,600 years ago, the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the Laurentide Ice Sheet into the Gulf of Mexico. Strong upwelling between 12,600 and about 10,000 years ago may be related to intensified trade winds resulting, in part, from cooler sea surface temperatures in the Caribbean and Gulf of Mexico. After about 10,000 years ago, upwelling intensity was reduced, though highly variable. A preliminary frequency domain analysis of the Holocene portion of the Cariaco Basin time series suggests that solar forcing may explain a significant component of the century-scale variability observed in the record of upwelling and trade wind strength.

Climatically sensitive eolian and hemipelagic deposition in the Cariaco Basin, Venezuela, over the past 578,000 years: Results from Al/Ti and K/Al

Al/Ti and K/Al ratios in bulk sediment are used to interpret wind-blown and hemipelagic sources of deposition to a 578 kyr record in the Cariaco Basin, Venezuela (Ocean Drilling Program site 1002). Graphical and cross-spectral analyses indicate that these ratios vary extremely closely with planktonic foraminiferal δ18O, with both ratios being significantly higher during interglacials and lower during glacials. K/Al indicates that during glacials the lower sea level that results in relative basin isolation increases the relative proportion of kaolinite derived from local rivers draining the relatively humid Venezuelan margin. Al/Ti decreases during glacials, suggesting greater proportions of eolian rutile sourced from the northern Sahara (due to increased wind strength and/or aridity). This interpretation is consistent with previous studies of the chemistry and mineralogy of Saharan-derived eolian matter in the Caribbean and with a mass balance determining the effect of changes in eolian rutile accumulation on the bulk sedimentary Al/Ti ratio.

Glacial/interglacial variations in production and nitrogen fixation in the Cariaco Basin during the last 580 kyr

The effect of sea level change on nutrient supply to the anoxic Cariaco demonstrates the fundamental importance of nitrogen (N2) fixation and phosphate to oceanic production. As N2 fixation produces biomass of low δ15N and has been reported to be an important component of the nitrogen cycle in the modern Cariaco Basin, we propose that it contributes to the light interglacial δ15N (∼2‰–3‰) values observed in the Ocean Drilling Program (ODP) site 1002 sediment record. During the glacials the sediments are bioturbated (oxic conditions) with low total organic carbon (TOC) contents and sedimentary δ15N values of ∼5‰, suggesting that nitrogen (N2) fixation contributed little to the N nutrition of Cariaco surface waters. The most plausible explanation for the inferred glacial/interglacial changes in N2 fixation in the Cariaco is that they have occurred in response to variations in the N/P ratio of the nutrient supply, driven by changes in denitrification.

Bundled turbidite deposition in the central Pandora Trough (Gulf of Papua) since Last Glacial Maximum : Linking sediment nature and accumulation to sea level fluctuations at millennial timescale

Since Last Glacial Maximum (23-19 ka), Earth climate warming and deglaciation occurred in two major steps (Bolling-Allerod and Preboreal), interrupted by a short cooling interval referred to as the Younger Dryas (12.5-11.5 ka B. P.). In this study, three cores (MV-33, MV-66, and MD-40) collected in the central part of Pandora Trough (Gulf of Papua) have been analyzed, and they reveal a detailed sedimentary pattern at millennial timescale. Siliciclastic turbidites disappeared during the Bolling-Allerod and Preboreal intervals to systematically reoccur during the Younger Dryas interval. Subsequent to the final disappearance of the siliciclastic turbidites a calciturbidite occurred during meltwater pulse 1B. The Holocene interval was characterized by a lack of siliciclastic turbidites, relatively high carbonate content, and fine bank-derived aragonitic sediment. The observed millennial timescale sedimentary variability can be explained by sea level fluctuations. During the Last Glacial Maximum, siliciclastic turbidites were numerous when the lowstand coastal system was located along the modern shelf edge. Although they did not occur during the intervals of maximum flooding of the shelf (during meltwater pulses 1A and 1B), siliciclastic turbidites reappear briefly during the Younger Dryas, an interval when sea level rise slowed, stopped, or perhaps even fell. The timing of the calciturbidite coincides with the first reflooding of Eastern Fields Reef, an atoll that remained exposed for most of the glacial stages.

Climate‐induced variations in productivity and planktonic ecosystem structure from the Younger Dryas to Holocene in the Cariaco Basin, Venezuela

A high-resolution molecular organic geochemical study of sediments in the anoxic Cariaco Basin indicates significant changes in primary productivity and planktonic community structure associated with the transition from the Younger Dryas to the Holocene. Variations in climate conditions over the past 12 14C kyr have induced large-scale changes in upwelling intensity, which directly affected levels of primary productivity as reflected in accumulation rates of bulk productivity proxies. Concentrations and accumulation rates of sterol and alkenone biomarkers have been used to identify how productivity changes affected the structure of the planktonic ecosystem. A shift in the dominant primary producer from diatoms (Younger Dryas) to coccolithophores (Holocene) is identified. If productivity and ecosystem variations like those identified in the tropical upwelling zone of the Cariaco Basin region, occur throughout the tropical oceans, they have the potential to affect global climate through perturbations in the biogeochemical cycle of carbon.

Late Quaternary climate change from δ18O records of multiple species of planktonic foraminifera: High‐resolution records from the Anoxic Cariaco Basin, Venezuela

Seasonal trade wind-induced upwelling along the southern margin of the Caribbean Sea occurs in response to the annual migration of the Intertropical Convergence Zone. Laminated, high deposition rate sediments of the Cariaco Basin, a small anoxic basin on the Venezuelan continental shelf, clearly record large changes in the past intensity of this upwelling. Because sediments of the Cariaco Basin are largely unbioturbated, they offer a natural opportunity to study the stable isotopic records of multiple planktonic foraminiferal taxa and to evaluate their sensitivity to both the modern hydrography and temporal changes in upwelling intensity and climate. Oxygen isotope data (δ18O) from four dominant foraminiferal taxa are presented for the time period covering the last 28 kyr. The δ18O data from Globigerina bulloides, after correction for nonequilibrium precipitation, are used as a monitor of sea surface conditions during the winter-spring upwelling season. The δ18O data from white Globigerinoides ruber are used as a measure of annual-average conditions in the near surface, while pink G. ruber data are consistent with use as an index of endmember conditions during the summer-fall nonupwelling season. Data from the deeper dwelling Neogloboquadrina dutertrei yield information on conditions near the base of the local thermocline. During the last glacial, δ18O data from G. ruber and generally reduced interspecific differences indicate cooling of surface waters over the Cariaco Basin by up to 4°C. This longer-term cooling does not appear to be related to changes in upwelling intensity along the coast but may instead reflect more regional cooling of the larger Caribbean. Superimposed on this pattern, between 12.6 and ∼10 ka, is a convergence of δ18O data between G. bulloides and N. dutertrei, implying much stronger upwelling during the last deglaciation. This scenario is consistent with other evidence for high productivity at this time. At ∼14 ka, a sharp δ18O depletion event observed in all taxa seems to have been produced by increased freshwater discharge to the southern Caribbean, suggesting either higher regional rainfall or the influence of glacial melting in the Andes. Minimum δ18O values of pink G. ruber around 6–7 ka record warmer summer sea surface temperatures and/or decreased salinity in the mid-Holocene.