Are the Ice Ages Driven by a Single Orbital Parameter?
Huybers (2007) argued that the connection between Earth's orbital variations and the growth and decay of continental ice sheets is far simpler than has heretofore been believed. The application of the Rayleigh R test to a carefully constructed (and not orbitally tuned) sediment core stack showed a consistent and statistically significant (at the 99% level) phase relationship between deglaciation and obliquity, but no strong relationship involving precession or eccentricity, though the null result in the case of precession was not robust as age--model uncertainties in the stack were large relative to the precessional period. However, the insolation forcing terms contain no direct precessional and little direct eccentricity forcing. Rather, the effect of these orbital parameters lies in a combined term, esin (ω), which does not have the same phase characteristics as either term alone. We therefore computed the R values for this term from a different, and in this case orbitally tuned, core stack (Lisieke and Raymo, 2005). The statistically significant results obtained by Huybers were reproduced (i.e. the orbital tuning has not changed the R values) while the esin (ω) term had statistical significance in the Late Pleistocene. As age-model uncertainties will always be large relative to the precessional period, we extended this analysis to the output of an ice sheet/climate model in which the time control is by definition exact (and of course is not orbitally tuned), and which reproduces the broad features of the Pleistocene delta--18O record (Tarasov and Peltier, 2002). The R values computed from this output agree qualitatively with those from the core stack. The esin (ω) term is not significant in the early Pleistocene, but begins to grow in value at about 1.4 Ma, becoming significant at about 1.2 Ma. Obliquity, while always significant, decreases in the late Pleistocene until it is approximately equal to the growing esin (ω) term at about 1 Ma. The R value for eccentricity alone also increases with time, becoming significant over the last 1 Ma, while the precession term is never significant at the 99% level. It seems clear that the ice age cycle's relationship to orbital forcing is not a simple one, and no one orbital parameter dominates.
Late Quaternary Glaciations in the Central Peruvian Andes (10°-11°S) and Evidence for a Link to Heinrich Events
Seven cosmogenic 10Be ages from a moraine in the Santuario Nacional Bosque de Piedras de Huayllay (BP) in the Western Cordillera of the central Peruvian Andes (10°59'S, 76°20'W, 4180-4200 masl) are consistent with 10Be ages on moraines in both the Eastern Cordillera (40-45 km to the east) and Nevado Jeulla Rajo (NJR) massif (10°00'S, 77°16'W) at the southern end of the Cordillera Blanca (150 km to the northwest). In the BP, 10Be ages are ~14-15 ka on four ignimbrite boulders, ~26 and ~20 ka on two quartz boulders, and ~45 ka on ignimbrite bedrock below the trimline in the valley wall. In the Eastern Cordillera bordering Lake Junin, the most extensive glaciations are >150 ka, but end moraines farther upvalley date to the local last glacial maximum (LLGM; 25-30 ka) and a late-glacial stillstand or readvance (14-18 ka). In NJR, 10Be ages indicate that the largest lateral moraines were deposited during similar intervals (27-32 ka and 14-18 ka). Avulsion of a glacial valley preserved an older, smaller pair of lateral moraines (56-65 ka) in NJR; correlative moraines were apparently not preserved in the Junin valleys. We have found no moraines in NJR that date to the global LGM (ca. 19-24 ka), but see some evidence for an advance ca. 40-48 ka. Outwash deposits (ca. 43-50 ka) located beyond the termini of NJR moraines are underlain by lodgement till that extends ca. 6 km across the Conococha Plain, suggesting that at least one older glaciation was far more extensive than any of the late Quaternary NJR advances dated by 10Be (ages calculated using CRONUS-Earth Online Calculator v. 2.2, Lal/Stone time-dependent scaling, and zero erosion). The timing of glacial advances in the central Peruvian Andes since 70 ka suggests a correlation to Heinrich events and associated southward shifts of the Intertropical Convergence Zone (ITCZ) in the Atlantic Ocean. We propose that Peruvian glaciers typically expanded when southward migration of the ITCZ resulted in increased precipitation in the tropical Andes during colder intervals.
Centennial to millennial climate variability during the Holocene over the extra-tropical hemispheres in a transient GCM simulation
On glacial-interglacial timescales lead-lag linkages between the climate of the northern and the southern hemispheres are evident based, for example, on the analysis of ice cores. Here we use a transient climate simulation forced with changes in orbital forcings to investigate the question if also during the relatively short period of the Holocene [externally forced] teleconnections between the hemispheres are evident. The climate simulation is carried out with the coupled atmosphere-ocean general circulation model ECHO-G for the period between 7000 years BP until present day. On millennial timescales the leading atmospheric annular circulation modes, the wintertime Arctic [AO] and Antarctic Oscillation [AAO] show clear trends. In the course of the Holocene the AO shows a negative trend, whereas the AAO shows a clear [opposite] positive trend. On multi-decadal to centennial timescales the annular modes are however not related to each other, underpinning the high degree of internal climate variability controlling the annular modes on these timescales. Annual near- surface temperature north and south of 45° North and South, respectively, show clear negative trends in the course of the Holocene. Over the extra-tropical Northern Hemisphere this trend is most likely caused by the pronounced positive summer-to-autumn orbitally induced insolation anomaly compared to present-day. Over the extra-tropical Southern Hemisphere this trend can not be explained by direct orbital forcing. Here internal multi-centennial changes in the circum-Antarctic sea ice variability can most likely explain the decrease in near- surface temperatures. On multi-decadal to centennial timescales near-surface temperatures over the extra- tropical Northern and Southern hemispheres also show positive correlations between +0.3 [multi-decadal] to +0.5 [centennial timescales]. Our results, especially the millennial changes for the near-surface temperature evolution over the Southern Hemisphere, therefore, point to the importance of internally generated climate feedbacks, such as the ice-albedo feedbacks, dampening externally induced forcing changes.
Dating the Little Ice Age Advance of Jakobshavn Isbrae, Greenland, Using Pro-glacial Lake Sediments.
The Greenland Ice Sheet's (GIS) largest and fastest outlet glacier, Jakobshavn Isbrae, is one of the most significant contributors to GIS mass loss, draining an estimated 6.5% of the GIS area (Rignot and Kanagaratnam, 2006). Jakobshavn Isbrae has retreated significantly since the Little Ice Age (LIA, ca. 1250- 1900; Csatho et al., 2008), and continues to exhibit rapid changes in velocity and ice calving front position (Joughin et al., 2004). However, it is unknown for how long Jakobshavn Isbrae was at or near its extensive LIA position because there is a lack of chronological control on the LIA advance phase. We collected sediment cores from lakes just beyond the LIA margin to constrain the time when the advancing glacier's silt-laden meltwater entered the lake basins. Sediment cores from South Oval and Ice Boom lakes (informal names), which no longer receive glacial meltwater from Jakobshavn Isbrae because it has retreated out of their catchments, contain gyttja/glacial-silt/gyttja sequences that represent their non-glacial/pro-glacial/non-glacial histories. One additional site, ice-dammed Lake Morten (informal name), completely drained sometime between 1985 and 2001 AD. Outcrops of laminated sediments in the lake basin overly an intact tundra landscape. Four AMS radiocarbon dates from macrofossils immediately below the LIA sediments from the three lake basins reveal that Jakobshavn Isbrae reached its LIA maximum extent between 530±10 and 370±60 cal yr BP (1400-1640 AD). Furthermore, the continuous nature of the LIA-sediment units in all sites indicates that Jakobshavn Isbrae remained at or near its LIA maximum position between 1400-1640 AD and into the 20th century. Finally, pre-LIA organic-rich sediments at all sites continue uninterrupted down to basal sediments deposited during regional deglaciation in the Early Holocene. AMS radiocarbon ages on macrofossils from basal sediments at all sites range from 7220±40 to 8130±60 cal yr BP. We therefore interpret that the GIS's maximum extent since deglaciation occured during the LIA, and that between deglaciation and the LIA, Jakobshavn Isbrae terminated farther inland than the present ice margin.
Paleoclimatic and Paleoceanographic Holocene Sedimentary Records in the Gulf of California - Eastern Pacific Ocean Interhemispheric Connections
Initial results of a study on the distribution, thickness and stratigraphy of the sedimentary sequences in the Gulf of California are presented. The Gulf is an elongated narrow young oceanic basin bordered by the Baja California peninsula and mainland Mexico. The Gulf extends over 1200 km across the Tropic of Cancer from the tropical to the temperate zones, surrounded by arid and semi-arid regions, including the Sonora-Mojave Desert. Paleoceanographic conditions are dominated by water exchange at the Gulf mouth and water masses changes along the Gulf. Tectonic basins reach down in excess of 3000 m depths and get shallower to the north. Here we focus on the Holocene sediment sequences in the southern sector, which contains several marginal and central anoxic basins that constitute rich archives of paleoclimatic and paleoenvironmental evolution for the past 3.6 Ma. In the mouth area, main sources of sediments are silicic volcanic and intrusive rocks in the Baja peninsula and mainland, including Los Cabos and Puerto Vallarta batholiths. Fine-grained eolian dusts, pluvial and biogenic sediments are present in the sediment cores in the Gulf basins such as La Paz, Alfonso, Carmen, Pescadero and Guaymas basins. Turbiditic currents and tephra deposits also occur in the cores. Paleoclimatic records show the influences of regional processes, including the ENSO and PDO signals marked by drought and increased precipitation phases. Relative distribution and thickness of sediments at the mouth of the Gulf correlate with bathymetry and location with respect to spreading center, transform faults and margins of the peninsula and mainland Mexico. Rock magnetic core scans and mineralogy at few locations are available, which allow inferences on sediment sources, transport and deposition processes, diagenesis, paleoceanographic and paleoclimatic evolution for the Holocene.
High-Resolution Mg/Ca Ratios in a Coralline Red Alga as a Proxy for Bering Sea Temperature Variations and Teleconnections
We present the first continuous high-resolution record of Mg/Ca variations within an encrusting coralline red alga of the species Clathromorphum nereostratum from Amchitka Island, Aleutian Islands. Mg/Ca ratios of individual growth increments were analyzed by measuring a single point electron microprobe transect yielding a resolution of 15 samples/year on average, generating a continuous record from 1830 to 1967 of algal Mg/Ca variations. Results show that Mg/Ca ratios in the high-Mg calcite skeleton display pronounced annual cyclicity and archive late spring to late fall sea surface temperature (SST) corresponding to the main season of algal growth. Mg/Ca values correlate well to local SST (ERSSTJun-Nov, 1902-1967; r = 0.73 for 5-year mean), as well as to an air temperature record from the same region. Our data correlate well to a shorter Mg/Ca record from a second site, corroborating the ability of the alga to reliably record regional environmental signals. In addition, Mg/Ca ratios relate well to a 29-year stable oxygen isotope time series measured on the same sample, which provides additional support for the use of Mg as a paleotemperature proxy in coralline red algae, that is, unlike stable oxygen isotopes, not influenced by salinity fluctuations. High spatial correlation to large-scale SST variability in the North Pacific is observed, with patterns of strongest correlation following the direction of major oceanographic features (i.e., the signature of the Alaska Current and the Alaskan Stream), which play a key role in the exchange of water masses between the North Pacific and the Bering Sea through Aleutian Island passages. The time series further displays significant teleconnections with the signature of the Pacific Decadal Oscillation in the northeast Pacific and the Atlantic Multidecadal Oscillation.
Late Quaternary Paleoclimate and Rainfall Regimes in the Sonora Desert: a Multi-proxy Study in the Sediments of Laguna Seca de San Felipe
We present the multi-proxy data from the sediments of the ephemeral Laguna Seca de San Felipe located at 31° N in the north-western Sonora Desert to infer the Late Quaternary paleoclimatic conditions and rainfall regimes. The laguna is present in a tectonic basin between San Pedro Martir (west) and San Felipe (east) Mountains. Although winter rainfall is dominant in the north-western Sonora, the north-south trending 2500 m high San Pedro Martir ranges and its western slope do not permit the north-western winter precipitation to bring inflow into the San Felipe basin. The laguna presently receives precipitation of 60 mm/yr during July-September from south-eastern winds bringing sediments from the 500 m high San Felipe Mountains. The special geographic condition of the laguna makes it an ideal site to see the influences of both winter and summer rainfall on its hydrological conditions. Transition from an aeolian to lacustrine regime occurred at ca.44.5 14C kyr BP. The lacustrine sediments during 44.5-12 14C kyr BP are characterized by lower erosion and sedimentation, lower magnetic susceptibility, presence of both Juniperus and Pinus pollens and sulphate mineral thenardite. The sediments of last 12 14C kyr BP are characterized by higher sedimentation and magnetic susceptibility and absence of pollens and thenardite and comprise intercalations of lacustrine and aeolian sediments. As the laguna is present in the rain shadow eastern flanks of the San Pedro Martir Mountains, we relate the lower sedimentation during 44.5-12 14C kyr BP to dominant winter storm tracks from the Pacific. During this period, Juniperus and Pinus pollens were transported to the basin from the higher elevations of San Pedro Mountains. The anoxic-humid conditions during 44.5-28.6 14C kyr BP is related to the mean position of winter storm track at 31° N. During this period, the lower temperature might have favoured the precipitation of sulphates. In the last 12 14C kyr BP, increasing erosion, higher rate of sedimentation and absence of both Juniperus and Pinus pollens in the basin were related to dominant summer circulations from the Gulf of California brining sediments from the San Felipe Mountain. This is supported by the varying total REE abundance, light REE, heavy REE fractions and Eu anomalies suggesting different provenances for sediments of the aeolian and both the lacustrine regimes.
Paleoecological Calibration In Central Guatemala: Modern Pollen Rain From Bryophyte Polsters And Surface Sediments
Paleoecology requires understanding the correspondences between modern pollen rain and local-regional vegetation, in order to develop accurate paleovegetation reconstructions. Paleoecology in Guatemala has been developed largely over decades in the northern lowlands in close relationship with Classic Maya archaeology, where paleoenvironmental reconstructions have been made mainly through the use of fossil pollen. Scarcity of calibration studies in the Mesoamerican region however remains evident; nevertheless, they are necessary to produce reliable reconstructions. We present calibration pollen data from two locations in Central Guatemala: Lachua Lowlands and Purulha Highlands. Pollen spectra were analyzed from surface sediments samples (SS) from a lake and a small pond in Lachua, a river floodplain and a lake shore in Purulha. Bryophyte polsters samples (BP) were collected from the interior of minimally disturbed forests in both Lachua (rain forest) and Purulha (cloud forest). Pollen spectra between SS and BP differed in both locations. Analysis per location indicates that SS were more similar for Purulha, as compared to Lachua. Combined analysis of locations indicates that SS from both locations were related to anemophilous taxa - great production of pollen quantities that has high dispersion capacities-. This provides evidence that the pollen signal from SS is probably more regional than local. BP from Lachua and Purulha differed notably in their pollen signal, each location containing local taxa, tropical and temperate respectively. Some temperate anemophilous taxa were better represented in Lachua than in Purulha. Purulha SS were similar, and contained more taxa related to disturbance and anemophilous taxa. The arboreal pollen (AP) to non-arboreal (NAP) ratio (AP/NAP) of both SS and BP corresponded with the tree- prevalent landscape in Lachua. The SS AP/NAP ratio represented the deforested landscape of the river floodplain and lake environments in Purulha, while for BP the extensive forest cover from the cloud forest was represented. Comparison of SS with BP provides foundation to understand better the pollen signal recorded in the sedimentary record, since BP pollen signal bring information about what is not represented. The calibration analysis that we present brings in-depth exploration of pollen-vegetation relationships and represents an important contribution for paleoecological research in Guatemala.