International Marine Global Change Study Research Articles

Rapid precipitation changes in the tropical West Pacific linked to North Atlantic climate forcing during the last deglaciation

The cause of rapid hydrological changes in the tropical West Pacific during the last deglaciation remains controversial. In order to test whether these changes were triggered by abrupt climate change events in the North Atlantic Ocean, variations in precipitation during the last deglaciation (18–10 ka) were extracted from proxy records of chemical weathering and terrigenous input in the western Philippine Sea (WPS). The evolution of chemical weathering and terrigenous input since 27 ka was reconstructed using the chemical index of alteration (CIA), elemental ratios (K/Al, TOC/TN and Ti/Ca), δ13Corg, terrigenous fraction abundance and flux data from International Marine Global Change Study Program (IMAGES) core MD06-3054 collected on the upper continental slope of eastern Luzon (northern Philippines). Sediment deposited during the Last Glacial Maximum (LGM) shows weathering equal to or slightly greater than Holocene sediment in the WPS. This unusual state of chemical weathering, which is inconsistent with lower air temperatures and decreased precipitation in Luzon during the LGM, may be due to reworking of poorly consolidated sediments on the eastern Luzon continental shelf during the LGM sea-level lowstand. Rapid changes in chemical weathering, characterized by higher intensity during the Heinrich event 1 (H1) and Younger Dryas (YD) and lower intensity during the Bølling-Allerød (B/A), were linked to rapid variations in precipitation in the WPS during the last deglaciation. The higher terrigenous inputs during the LGM relative to those of the Holocene were controlled by sea-level changes rather than precipitation. The terrigenous inputs show a long-term decline during the last deglaciation, punctuated by brief spikes during the H1 and YD related to sea-level rises and rapid precipitation changes in the WPS, respectively. The proxy records of chemical weathering and terrigenous input from eastern Luzon suggest high rainfall during the H1 and YD events, consistent with inferred rainfall patterns based on Fe/Ca records from offshore Mindanao. Rapid precipitation changes in the WPS did not coincide with migrations of the Intertropical Convergence Zone (ITCZ) but, rather, were related to state shifts of the El Niño-Southern Oscillation (ENSO) during the last deglaciation. Based on proxy records and modeling results, we argue that the Atlantic meridional overturning circulation (AMOC) controlled rapid precipitation changes in the tropical West Pacific through zonal shifts of ENSO or meridional migration of the ITCZ during the last deglaciation. Our findings highlight the dominant role of the North Atlantic Ocean in the tropical hydrologic cycle during the last deglaciation.

Bathyal records of enhanced silicate erosion and weathering on the exposed Luzon shelf during glacial lowstands and their significance for atmospheric CO2 sink

We present a new high-resolution multiproxy data set of mass accumulation rates, Sr-Nd isotopes, as well as major and trace elements for the siliciclastic sediment fraction from International Marine Global Change Study Core MD06-3052, located on the continental slope of the western Philippine Sea. We integrate our new data with published grain sizes and sea surface temperatures from the same core, as well as with Equatorial Pacific sea level, and East Asian summer monsoon precipitation, in order to constrain at high-resolution changes in physical erosion and chemical weathering intensities on Luzon, and sediment source-to-sink processes. We assess the potential significance of chemical weathering of arc silicates in regulating global atmospheric CO2 since 156kyr BP. Sr-Nd isotopes show that the siliciclastic sediments were dominantly sourced from volcanic rocks exposed on Luzon (~68–100%), with a lesser contribution from Asian dust (~0–32%). Different indices indicate that stronger physical erosion and chemical weathering occurred during Marine Isotope Stage (MIS) 6 (130–156kyr BP), as well as in the latter stage of MIS 3 and MIS 2 (14–40kyr BP). The large sea-level lowstands and associated significant exposure of continental shelf in the western Philippine Sea during these two cold periods should favor physical erosion and chemical weathering of unconsolidated sediments on the exposed shelf. Furthermore, we notice the relatively good coherence between chemical weathering intensities on Luzon and global atmospheric CO2 concentrations over these cold intervals. We suggest that strengthening of chemical weathering of silicates on Luzon and other tropical arcs (within 20° of the Equator) during the Quaternary glacial sea-level lowstands may significantly contribute to the lowering of atmospheric CO2 concentrations during ice ages. We estimate that a significant fraction, up to ~16% (i.e., ~8ppmv), of all atmospheric CO2 ultimately sequestered by silicate weathering may be processed through an area corresponding to only ~1% of the exorheic drainage area worldwide.

西太平洋におけるIMAGES (International Marine Global Change Study)コアを用いた高時間解像度の環境復元の意義( 西太平洋におけるIMAGESコアを用いた高時間解像度の環境復元)

西太平洋におけるIMAGES (International Marine Global Change Study)コアを用いた高時間解像度の環境復元の意義( 西太平洋におけるIMAGESコアを用いた高時間解像度の環境復元)

Variations in the western Pacific warm pool across the mid-Pleistocene: Evidence from oxygen isotopes and coccoliths in the West Philippine Sea

We present planktonic foraminifera oxygen isotope and Florisphaera profunda abundance data from the International Marine Global Change Study Program (IMAGES) Core MD06-3050, which was collected in the West Philippine Sea on the margin of the Western Pacific Warm Pool (WPWP). Our records reveal marked changes in the upper water structure in the Kuroshio source region as early as 1.5Ma, continuing through the mid-Pleistocene climate transition (MPT). These changes are evidenced by reconstructions of the thermocline (based on planktonic foraminiferal oxygen isotope differences) and the nutricline (based on the relative abundance of the coccolithophore species Florisphaera profunda) in the Kuroshio source region. The evolution of the thermocline/nutricline featured three prominent phases: 1) phase I (1.5–1.1Ma), which was characterized by low-amplitude variations in a shallow thermocline; 2) phase II (1.1–0.8Ma), which was characterized by high-amplitude variations in a relatively shallow thermocline/nutricline; and 3) phase III (0.8–0.5Ma), which was characterized by low-amplitude variations in a deep thermocline/nutricline. Combined with the zonal sea surface temperature gradient in the tropical Pacific, the evolution of the upper water structure can be linked to tropical climatic changes across the MPT, including the intensification of atmospheric circulation and a gradual cessation of the long-term El Niño-like state. The upper water structure of the WPWP was asymmetric before 0.8Ma, with a shallow thermocline/nutricline along the northern margin and a deep thermocline/nutricline in the center of the WPWP and South China Sea (SCS). The modern WPWP was established by approximately 0.8Ma, and the upper water structure has changed synchronously as a whole since that time. A comparison of the δ18O values derived from Globigerinoides ruber tests from the MD06–3050 core with those from other cores in the western Pacific suggests that the lower glacial/interglacial G. ruber δ18O values in MD06-3050 before 0.8Ma could be related to the enhancement of the WPWP and to changes in atmospheric circulation patterns.

Quantitative estimates of Asian dust input to the western Philippine Sea in the mid‐late Quaternary and its potential significance for paleoenvironment

AbstractWe present a new high‐resolution multiproxy data set of Sr‐Nd isotopes, rare earth element, soluble iron, and total organic carbon data from International Marine Global Change Study Core MD06‐3047 located in the western Philippine Sea. We integrate our new data with published clay mineralogy, rare earth element chemistry, thermocline depth, and δ13C differences between benthic and planktonic foraminifera, in order to quantitatively constrain Asian dust input to the basin. We explore the relationship between Philippine Sea and high‐latitude Pacific eolian fluxes, as well as its significance for marine productivity and atmospheric CO2 during the mid‐late Quaternary. Three different indices indicate that Asian dust contributes between ∼15% and ∼50% to the detrital fraction of the sediments. Eolian dust flux in Core MD06‐3047 is similar to that in the polar southern Pacific sediment. Coherent changes for most dust flux maximum/minimum indicate that dust generation in interhemispheric source areas might have a common response to climatic variation over the mid‐late Quaternary. Furthermore, we note relatively good coherence between Asian dust input, soluble iron concentration, local marine productivity, and even global atmospheric CO2 concentration over the entire study interval. This suggests that dust‐borne iron fertilization of marine phytoplankton might have been a periodic process operating at glacial/interglacial time scales over the past 700 ka. We suggest that strengthening of the biological pump in the Philippine Sea, and elsewhere in the tropical western Pacific during the mid‐late Quaternary glacial periods may contribute to the lowering of atmospheric CO2 concentrations during ice ages.

Persistently strong Indonesian Throughflow during marine isotope stage 3: evidence from radiogenic isotopes

The Indonesian Throughflow (ITF) connects the western Pacific Ocean with the eastern Indian Ocean, thus forming one of the major near surface current systems of the global thermohaline circulation. The intensity of the ITF has been found to be sensitive to changes in global ocean circulation, fluctuations in sea level, as well as to the prevailing monsoonal conditions of the Indonesian Archipelago and NW Australia. This study presents the first reconstruction of ITF dynamics combining radiogenic isotope compositions of neodymium (Nd), strontium (Sr), and lead (Pb) of the clay-size detrital fraction to investigate changes in sediment provenance, and paleo seawater Nd signatures extracted from the planktonic foraminifera and authigenic Fe–Mn oxyhydroxide coatings of the marine sediments focussing on marine isotope stage 3 (MIS3). Sediment core MD01-2378 was recovered within the framework of the International Marine Global Change Study (IMAGES) and is located in the area of the ITF outflow in the western Timor Sea (Scott Plateau, 13° 04.95′ S and 121° 47.27′ E, 1783 m water depth). In order to produce reliable seawater signatures, several extraction methods were tested against each other. The results of the study show that at this core location the extraction of surface water Nd isotope compositions from planktonic foraminifera is complicated by incomplete removal of contributions from Fe–Mn oxyhydroxides carrying ambient bottom water signatures. The bottom water Nd isotope signatures reliably obtained from the sediment coatings (average εNd = −5.0) document an essentially invariable water mass composition similar to today throughout the entire MIS3. The radiogenic Nd, Sr, and Pb isotope records of the clay-sized detrital fraction suggest that the Indonesian Archipelago rather than NW Australia was the main particle source at the location of core MD01-2378, and thus indicating a persistently strong ITF during MIS3. Furthermore, the variations of the detrital radiogenic isotopes are shown to be more sensitive to changes in circulation and document a somewhat enhanced ITF intensity during the early part of MIS3 until 47.4ka compared with the remaining MIS3.

An ocean–ice coupled response during the last glacial: a view from a marine isotopic stage 3 record south of the Faeroe Shetland Gateway

Abstract. The rapid climatic variability characterising the Marine Isotopic Stage (MIS) 3 (~60–30 cal ka BP) provides key issues to understand the atmosphere–ocean–cryosphere dynamics. Here we investigate the response of sea-surface paleoenvironments to the MIS3 climatic variability through the study of a high resolution oceanic sedimentological archive (core MD99-2281, 60°21' N; 09°27' W; 1197 m water depth), retrieved during the MD114-IMAGES (International Marine Global Change Study) cruise from the southern part of the Faeroe Bank. This sector was under the proximal influence of European ice sheets (Fennoscandian Ice Sheet to the East, British Irish Ice Sheet to the South) during the last glacial and thus probably responded to the MIS3 pulsed climatic changes. We conducted a multi-proxy analysis of core MD99-2281, including magnetic properties, x-ray fluorescence measurements, characterisation of the coarse (>150 μm) lithic fraction (grain concentration) and the analysis of selected biogenic proxies (assemblages and stable isotope ratio of calcareous planktonic foraminifera, dinoflagellate cyst – e.g. dinocyst – assemblages). Results presented here are focussed on the dinocyst response, this proxy providing the reconstruction of past sea-surface hydrological conditions, qualitatively as well as quantitatively (e.g. transfer function sensu lato). Our study documents a very coherent and sensitive oceanic response to the MIS3 rapid climatic variability: strong fluctuations, matching those of stadial/interstadial climatic oscillations as depicted by Greenland ice cores, are recorded in the MD99-2281 archive. Proxies of terrigeneous and detritical material suggest increases in continental advection during Greenland Stadials (including Heinrich events), the latter corresponding also to southward migrations of polar waters. At the opposite, milder sea-surface conditions seem to develop during Greenland Interstadials. After 30 ka, reconstructed paleohydrological conditions evidence strong shifts in SST: this increasing variability seems consistent with the hypothesised coalescence of the British and Fennoscandian ice sheets at that time, which could have directly influenced sea-surface environments in the vicinity of core MD99-2281.

Mindanao Dome variability over the last 160 kyr: Episodic glacial cooling of the West Pacific Warm Pool

We present sea surface, upper thermocline, and benthic δ18O data, as well as temperature and paleoproductivity proxy data, from the International Marine Global Change Study Program (IMAGES) Core MD06‐3067 (6°31′N, 126°30′E, 1575 m water depth), located in the western equatorial Pacific Ocean within the flow path of the Mindanao Current. Our records reveal considerable glacial‐interglacial and suborbital variability in the Mindanao Dome upwelling over the last 160 kyr. Dome activity generally intensified during glacial intervals resulting in cooler thermocline waters, whereas it substantially declined during interglacials, in particular in the early Holocene and early marine oxygen isotope stage (MIS) 5e, when upwelling waters did not reach the thermocline. During MIS 3 and MIS 2, enhanced surface productivity together with remarkably low SST and low upper ocean thermal contrast provide evidence for episodic glacial upwelling to the surface, whereas transient surface warming marks periodic collapses of the Mindanao Dome upwelling during Heinrich events. We attribute the high variability during MIS 3 and MIS 2 to changes in the El Niño Southern Oscillation state that affected boreal winter monsoonal winds and upper ocean circulation. Glacial upwelling intensified when a strong cyclonic gyre became established, whereas El Niño–like conditions during Heinrich events tended to suppress the cyclonic circulation, reducing Ekman transport. Thus, our findings demonstrate that variations in the Mindanao Dome upwelling are closely linked to the position and intensity of the tropical convection and also reflect far‐field influences from the high latitudes.

Evidence for Indonesian Throughflow slowdown during Heinrich events 3–5

We present sea surface and upper thermocline temperature records (60–100 year temporal resolution) spanning marine isotope stage 3 (∼24–62 ka B.P.) from International Marine Global Change Study core MD01‐2378 (121°47.27′E and 13°04.95′S; 1783 m water depth) located in the outflow area of the Indonesian Throughflow within the Timor Sea. Stable isotopes and Mg/Ca of the near‐surface‐dwelling planktonic foraminifer Globigerinoides ruber (white) and the upper thermocline–dwelling Pulleniatina obliquiloculata reveal rapid changes in the thermal structure of the upper ocean during Heinrich events. Thermocline warming and increased δ18Oseawater (P. obliquiloculata record) during Heinrich events 3, 4, and 5 reflect weakening of the relatively cool and fresh thermocline flow and reduced export of less saline water from the North Pacific and Indonesian Seas to the tropical Indian Ocean. Three main factors influenced Indonesian Throughflow variability during marine isotope stage 3: (1) global slowdown in thermohaline circulation during Heinrich events triggered by Northern Hemisphere cooling, (2) increased freshwater export from the Java Sea into the Indonesian Throughflow controlled by rising sea level from ∼60 to 47 ka, and (3) insolation‐related changes in the Australasian monsoon with associated migration of hydrological fronts between Indian Ocean– and Indonesian Throughflow–derived water masses at ∼46–40 ka.

Paleomagnetic field variation with strong negative inclination during the Brunhes chron at the Banda Sea, equatorial southwestern Pacific

We reconstruct the earth magnetic field in the Brunhes epoch at the Banda Sea by studying the paleomagnetic data from core MD012380, collected during the International Marine Global Change Study (IMAGES) VII Cruise in 2001. Magnetic analysis is carried out for whole core with a sampling spacing of 1cm by using u-channel. Magnetic susceptibility (χ), nature remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) are measured in our paleomagnetic experiment. Results show the low latitude geomagnetic field variation at the Banda Sea during the last ∼820kyr. Except for the Brunhes/Matuyama boundary (BMB), there is no clear signal of reverse events in paleo- inclination and paleo-declination patterns. However, the synthetic paleointensity curve displays the asymmetrical saw-tooth pattern that can be used for determining reverse events, and shows a maximum intensity drop at the BMB. The characteristics of paleointensity provide a useful tool to identify reverse signals and improve the difficulties from only using inclination and declination patterns, especially at low latitude. With the help of paleointensity, inclination and declination, we have identified five reverse events. Furthermore if we consider the secular variation effect, we think that the strong negative inclination observed in our study may be the zonal time-averaged field with paleo secular variation, rather than non-dipole effect within the Brunhes epoch.

PAGES: Always bottom up

In a recent article titled “Back to the Future” (Eos, 16 March, p. 107), L. C. Witton lays out the goals of IGBP‐PAGES for the next few years, noting that, “PAGES is aiming to become a truly bottom‐up organization that is driven by the insights of individual scientists…”In fact, PAGES has always been a truly bottom‐up organization, and this statement unfortunately fosters the view that it has been otherwise. Those who promote such a view choose to overlook the countless workshops that PAGES has organized, largely at the suggestion of those “at the bottom,” and the numerous publications that have resulted from these meetings. These include reports and recommendations on inter alia: IDEAL—An International Decade for the East African Lakes, the PAGES/CLIVAR Intersection, High Resolution Records of Past Climate from Monsoon Asia, Paleomonsoons in Africa, IMAGES—International Marine Global Change Study, PANASH‐Paleoclimates of the Northern and Southern Hemispheres, GNIP—Global Network for Isotopes in Precipitation, Continental Drilling for Paleoclimate Records, Climate Impact of Explosive Volcanism, International Trans‐Antarctic Scientific Expedition (ITASE), ARTS—Annual Records of Tropical Systems and the Paleoclimate Modelling Intercomparison Project (PMIP) (see http://www.pages.unibe.ch).

Progress in global lake drilling holds potential for global change research

During the past decade, numerous international investigations of past global change have focused on particular time intervals, or “Time Streams,” suggested by the Past Global Changes (PAGES) Project of the International Geosphere‐Biosphere Programme (IGBP). Time Stream 1 encompasses the last 2000 years, and Time Stream 2 encompasses at least the last 250,000 years. Geographically many of these studies have been grouped into north‐south transects of continental global change records known as the Pole‐Equator‐Pole (PEP) transects [Bradley et al., 1995]. These continental transects have been complemented by the study of marine records included in the International Marine Global Change Study (IMAGES) transects and high‐latitude ice core records such as those from the U.S. Greenland Ice Sheet Project 2 (GISP2) and the European Greenland Ice‐core Project (GRIP).

A Summary of Glaciation

CLIMATE SCIENCE The EPILOG (Environmental Processes of the Ice Age: Land, Oceans, Glaciers) program originated in 1998, under the aegis of the IMAGES (International Marine Global Change Study) program of IGBP/PAGES (International Geosphere-Biosphere Programme/Past Global Changes). It was charged with developing a comprehensive description of Earth during the last glacial maximum (LGM) 21,000 years ago. A primary goal was to explore the transitions into and out of a full glacial state by updating the pictures of sea surface temperature, glacial ice distribution, and albedo provided in 1981 by the landmark study CLIMAP. The EPILOG 2000 workshop summarized the current knowledge about continental ice sheets and sea level immediately before, during, and after the LGM. A collection of papers drawn from the workshop addresses four main issues: (i) the extent, volume, and evolution of glaciers and ice sheets; (ii) the magnitude of global sea level change caused by the growth and decay of ice sheets; (iii) the isostatic response to the global redistribution of ice and water; and (iv) the relation between ice sheets and global climate change. These papers discuss geological reconstructions of ice sheet extent, records of sea level change, ice sheet modeling, geophysical models of glacial isostatic adjustment, and geochemical proxies of ice volume. From this body of work, several important conclusions emerge. First, sea level at the LGM was 118 to 135 meters below the modern value, bracketing the lower bound for sea level drop of 127 meters and much less than the maximum value of 163 meters estimated by CLIMAP. Second, although new estimates of the margins of ice sheets agree in most cases with the boundaries drawn by CLIMAP, the amount and distribution of ice were substantially less than the CLIMAP estimates. Finally, some or all of the ice sheets must have been thinner than the minimum value given by CLIMAP. — HJS Quat. Sci. Rev. 21 , 1 (2002).