Alkenone-derived Sea Surface Temperature Research Articles

A multiproxy reconstruction of Asian winter monsoon variability since the last glacial from southeast offshore Hainan Island, South China Sea

The evolution of East Asian winter monsoon (EAWM) in tropical oceans has been a key issue in the climatic coupling between high- and low-latitudes. This study aims to investigate the EAWM variability using multiproxy of sediment core XB19S40 in the southeast offshore of Hainan Island, South China Sea. Alkenone derived sea surface temperature (SST), sediment grain size, and elemental ratios show consistent changes over the last 20.7 ka. There were synchronous abrupt changes in the SST, Zr/Rb, K/Ti, and End-members (EMs) during the Bølling-Allerød warming period, Younger Dryas and 8.2 ka cooling events. The Zr/Rb and EM2 (18.7–84.6 μm, 40.1 μm) changes reflect the EAWM and are highly coincident with records in high latitudes. These changes suggest that the EAWM gradually weakened from the last glacial maximum to early Holocene, experienced minor strengthening in the late Holocene but has diminished since 1 ka. In contrast, the indicators K/Ti and EM1 (0.2–18.7 μm, 9.9 μm) suggest that the East Asian summer monsoon is in the opposite phase. Spectral analysis demonstrates that the intensity of East Asian monsoons in the SCS were mainly affected by solar activity on a centennial-scale. The results of this study provide new insight into the complexity of the EAWM system and emphasize the importance of understanding multiple components of the climate system.

Upper-ocean temperature characteristics in the subantarctic southeastern Pacific based on biomarker reconstructions

Abstract. As remnants of living organisms, alkenones and isoprenoid glycerol dialkyl glycerol tetraether lipids (isoGDGTs) are widely used biomarkers for determining ocean water temperatures from the past. The organisms that these proxy carriers stem from are influenced by a number of environmental parameters, such as water depth, nutrient availability, light conditions, or seasonality, which all may significantly bias the calibration to ambient water temperatures. Reliable temperature determinations thus remain challenging, especially in higher latitudes and for undersampled regions. We analyzed 33 sediment surface samples from the southern Chilean continental margin and the Drake Passage for alkenones and isoGDGTs and compared the results with gridded instrumental reference data from the World Ocean Atlas 2005 (WOA05) and previously published data from an extended study area covering the central and western South Pacific towards the Aotearoa / New Zealand continental margin. We show that for alkenone-derived sea surface temperatures (SSTs), the widely used global core-top calibration of Müller et al. (1998) yields the smallest deviation of the WOA05-based SSTs. On the contrary, the calibration of Sikes et al. (1997), determined for higher latitudes and summer SSTs, overestimates modern WOA05-based SSTs in both the annual mean and summer. Our alkenone SSTs show a slight seasonal shift of ∼ 1 ∘C at the southern Chilean margin and up to ∼ 2 ∘C in the Drake Passage towards austral summer SSTs. Samples in the central South Pacific, on the other hand, reflect an annual mean signal. We show that for isoGDGT-based temperatures, the subsurface calibration of Kim et al. (2012a) best reflects temperatures from the WOA05 in areas north of the Subantarctic Front (SAF). Temperatures south of the SAF are, in contrast, significantly overestimated by up to 14 ∘C, irrespective of the applied calibration. In addition, we used the GDGT [2]/[3] ratios, which give an indication of the production depth of the isoGDGTs and/or potential influences from land. Our samples reflect a subsurface (0–200 m water depth) rather than a surface (0–50 m water depth) signal in the entire study area and show a correlation with the monthly dust distribution in the South Pacific, indicating terrigenous influences. The overestimation of isoGDGT surface and subsurface temperatures south of the SAF highlights the need for a reassessment of existing calibrations in the polar Southern Ocean. Therefore, we suggest a modified Southern Ocean tetraether index (TEX86)-based calibration for surface and subsurface temperatures, which shows a lower temperature sensitivity and yields principally lower absolute temperatures, which align more closely with WOA05-derived values and also OH–isoGDGT-derived temperatures.

A comparison of orbital-resolution, Late Pleistocene Alkenone and foraminiferal assemblage-based sea surface temperature reconstructions from the Southwest Pacific

Global and regional reconstructions of past climate conditions often incorporate sea surface temperature (SST) estimates from multiple proxies because not every paleotemperature proxy is applicable in all geographic locations. This practice of assimilating estimates from different proxies in global or regional temperature syntheses makes the implicit assumption that estimates derived from different proxies can be meaningfully intercompared. However, evidence to support the validity of this assumption is limited. Using paleotemperature data from sediments collected from ODP Site 1125 in the Southwest Pacific, we conduct a ∼1 Myr, orbital-scale SST proxy comparison of a recently published alkenone-derived SST record with a previously published foraminiferal assemblage-based SST record. These alkenone and foraminiferal assemblage SST datasets show strong structural similarity and yield remarkably similar estimates for basic climate metrics, including mean, median, standard deviation, and range. Statistical analysis indicates that the correlation between the two SST records is highly significant. In the spectral domain, the records share the same dominant 100 kyr beat, are coherent and in phase with each other at this frequency, and have the same coherence and phase relationship with benthic foraminiferal δ18O. Results from this work demonstrate that these two proxies would yield very similar estimates for the paleoclimate metrics most commonly used in empirical paleoclimate reconstructions that seek to document the evolution of climate over this interval. However, significant disparities between SST estimates derived from the two proxies exist for some time periods, particularly during glacial and interglacial extrema. This comparison suggests that treating estimates from these proxies as equivalent in studies that focus on short time windows (e.g. a few thousand to tens-of-thousands of years), particularly in investigations that seek to characterize glacial or interglacial extrema, could be potentially problematic. However, the sensitive location of Site 1125, just north of the Subtropical Front, likely accentuates the difference between temperature estimates from these proxies, which may be attenuated in other oceanographic settings. We attribute the discrepancies between the two SST records to two main causes: seasonal leakages of cold water across the Subtropical Front during glacial extrema and differential seasonality of maximum alkenone and foraminiferal production.

Trends and centennial-scale variability of surface water temperatures in the North Atlantic during the Holocene

Two sediment cores retrieved off North Iceland (western Nordic Seas) and on the eastern flank of Reykjanes Ridge (Iceland Basin) were analyzed to generate high-resolution alkenone-derived sea surface temperature (SST) records to investigate North Atlantic Ocean circulation changes during the Holocene. Early Holocene SSTs off North Iceland were unstable (10 ± 1 °C) and 3 °C warmer than today reflecting active northward heat transport of the Atlantic Meridional Overturning Circulation (AMOC) interrupted by intermittent Polar Waters incursions onto the North Icelandic shelf. The Holocene thermal optimum occurred synchronously east of Reykjanes Ridge, with a mean value of 11.5 °C (±0.5 °C) similar to today, consistent with a sustained influence of AMOC. Both records indicate that the circulation across the North Atlantic intensified between 8000 and 7000 yr BP. Thereafter, SSTs in the two basin sites broadly depict opposing trends and centennial-scale oscillations and a notable cooling at ∼5300 yr BP that coincides with Bond 4 event and the temporary collapse of the deep-water circulation. From 2500 yr BP onwards, SSTs in the Iceland Basin and the western Nordic Seas diverge leading to a marked cooling/warming dipole resulting in a temperature difference today of 4.5 °C. We show that SST trends and centennial-scale variability reflect variations of the subpolar gyre (SPG) circulation linked to drifting ice events and convection changes in the Labrador and Nordic Seas.

A composite Pliocene record of sea surface temperature in the central Mediterranean (Capo Rossello composite section – South Sicily)

The Pliocene (5.33–2.58 Ma) was the last period in the Earth's history when atmospheric CO2 concentrations were similar to today (~400 ppm) and global mean temperatures were significantly warmer. In the Mediterranean, the Pliocene, and more particularly the Piacenzian (3.6–2.58 Ma), has been intensively studied but less is known about the Early Pliocene climate. This study aims at filling this gap with a high-resolution alkenone-derived sea surface temperature (SST) record of the Zanclean epoch obtained from the composite Capo Rossello section (South Sicily). This new dataset is combined with the previously published records of Beltran et al. (2011), Herbert et al. (2015) and Herbert et al. (2018), to document SST changes during major regional and global oceanographic re-organizations such as the connection with the Atlantic basin. Our SST record highlights the key transition between the warm and unstable conditions during the Zanclean and the more stable and cooler Piacenzian, around 3.8 Ma ago. We suggest that this transition results from the intensification of the Mediterranean outflow and the establishment of the Northern Hemisphere glaciation. The combined effect of these mechanisms would be responsible for cooler, more seasonal and possibly dryer conditions in the Mediterranean.

Climate variability during MIS 20–18 as recorded by alkenone-SST and calcareous plankton in the Ionian Basin (central Mediterranean)

This study shows the first Mediterranean high-resolution record of alkenone-derived sea surface temperature (SST) in the marine sediments outcropping at the Ideale section (IS) (southern Italy, central Mediterranean) from late marine isotope stage (MIS) 20 - through early MIS 18. The SST pattern evidences glacial-interglacial up to submillennial-scale temperature variation, with lower values (~13 °C) in late MIS 20 and substage 19b, and higher values (up to 21 °C) in MIS 19c and in the interstadials of MIS 19a. The SST data are combined with the new calcareous plankton analysis and the available, chronologically well-constrained carbon and oxygen isotope records in the IS. The multi-proxy approach, together with the location of the IS near the Italian coasts, the lower circalittoral-upper bathyal depositional setting, and high sedimentation rate allow to document long-and short-term paleoenvironmental modifications (sea level, rainfall, inorganic/organic/fresh water input to the basin), as a response to regional and global climate changes. The combined proxies reveal the occurrence of a terminal stadial event in late MIS 20 (here Med-HTIX), and warm-cold episodes (here Med-BATIX and Med-YDTIX) during Termination IX (TIX), which recall those that occurred through the last termination (TI). During these periods and the following ghost sapropel layer (insolation cycle 74, 784 ka) in the early MIS 19, high frequency internal changes are synchronously recorded by all proxies. The substage MIS 19c is warm but quite unstable, with several episodes of paleoenvironmental changes, associated with fluctuating tropical-subtropical water inflow through the Gibraltar Strait, variations of the cyclonic regime in the Ionian basin, and the southward shift of westerly winds and winter precipitation over southern Europe and Mediterranean basin. Three high-amplitude millennial-scale oscillations in the patterns of SST and calcareous plankton key taxa during MIS 19a are interpreted as linked to changes in temperature as well as in salinity due to periodical water column stratification and mixing.The main processes involved in the climate variability include changes in oceanographic exchanges through the Gibraltar Strait during modulations of Atlantic meridional overturning circulation and/or variations in atmospheric dynamics related to the influence of westerly and polar winds acting in the paleo-Ionian basin. A strong climate teleconnection between the North Atlantic and Mediterranean is discussed, and a prominent role of atmospheric processes in the central Mediterranean is evidenced by comparing data sets at the IS with Italian and extra-Mediterranean marine and terrestrial records.

Contrasting late-glacial paleoceanographic evolution between the upper and lower continental slope of the western South Atlantic

Abstract. The number of sedimentary records collected along the Brazilian continental margin has increased significantly in recent years, but relatively few are located in shallow waters and register paleoceanographic processes in the outer shelf–middle slope prior to 10–15 ka. For instance, the northward flow up to 23–24∘ S of cold and fresh shelf waters sourced from the Subantarctic region is an important feature of current hydrodynamics in the subtropical western South Atlantic Ocean, and yet limited information is available for the long-term changes of this system. Herein, we considered a suite of organic and inorganic proxies – alkenones-derived sea surface temperature (SST), δD-alkenones, δ18O of planktonic foraminifera, and ice-volume free seawater δ18OIVF−SW – in sediment from two cores (RJ-1501 and RJ-1502) collected off the Rio de Janeiro Shelf (SE Brazilian continental shelf) to shed light on SST patterns and relative salinity variations since the end of the last glacial cycle in the region and the implications of these processes over a broader spatial scale. The data indicate that, despite the proximity (∼40 km apart) of both cores, apparently contradictory climatic evolution occurred at the two sites, with the shallower (deeper) core RJ-1501 (RJ-1502) showing consistently cold (warm) and fresh (salt) conditions toward the Last Glacial Maximum (LGM) and last deglaciation. This can be reconciled by considering that the RJ-1501 core registered a signal from mid- to high latitudes on the upper slope off Rio de Janeiro represented by the influence of the cold and fresh waters composed of Subantarctic Shelf Water and La Plata Plume Water transported northward by the Brazilian Coastal Current (BCC). The data from core RJ-1502 and previous information for deep-cores from the same region support this interpretation. In addition, alkenone-derived SST and δ18OIVF−SW suggest a steep thermal and density gradient formed between the BCC and Brazil Current (BC) during the last climate transition which, in turn, may have generated perturbations in the air–sea heat flux with consequences for the regional climate of SE South America. In a scenario of future weakening of the Atlantic Meridional Overturning Circulation, the reconstructed gradient may become a prominent feature of the region.

Late Pliocene Cordilleran Ice Sheet development with warm northeast Pacific sea surface temperatures

Abstract. The initiation and evolution of the Cordilleran Ice Sheet are relatively poorly constrained. International Ocean Discovery Program (IODP) Expedition 341 recovered marine sediments at Site U1417 in the Gulf of Alaska (GOA). Here we present alkenone-derived sea surface temperature (SST) analyses alongside ice-rafted debris (IRD), terrigenous, and marine organic matter inputs to the GOA through the late Pliocene and early Pleistocene. The first IRD contribution from tidewater glaciers in southwest Alaska is recorded at 2.9 Ma, indicating that the Cordilleran Ice Sheet extent increased in the late Pliocene. A higher occurrence of IRD and higher sedimentation rates in the GOA during the early Pleistocene, at 2.5 Ma, occur in synchrony with SSTs warming on the order of 1 ∘C relative to the Pliocene. All records show a high degree of variability in the early Pleistocene, indicating highly efficient ocean–climate–ice interactions through warm SST–ocean evaporation–orographic precipitation–ice growth mechanisms. A climatic shift towards ocean circulation in the subarctic Pacific similar to the pattern observed during negative Pacific Decadal Oscillation (PDO) conditions today occurs with the development of more extensive Cordilleran glaciation and may have played a role through increased moisture supply to the subarctic Pacific. The drop in atmospheric CO2 concentrations since 2.8 Ma is suggested as one of the main forcing mechanisms driving the Cordilleran glaciation.

(In)coherent multiproxy signals in marine sediments: Implications for high-resolution paleoclimate reconstruction

Accurate chronologies are the backbone of paleoclimate research, yet for marine sedimentary records the same age model is often applied to co-eval components that may have experienced different pre-depositional histories, leading to aliasing in corresponding proxy signals. Here we demonstrate 14C age discrepancies among several proxy carriers and use them to highlight spatio-temporal disparities among different components of the sediment. Total organic carbon (TOC), alkenones, and alkanoic (fatty) acids have older ages than co-occurring planktonic foraminifera in corresponding depth intervals of a sediment core retrieved from the so-called ‘Shackleton Sites’ on the southwest Iberian margin off Portugal. Temporal differences are explained by the addition of pre-aged material that is transported to the site by lateral advection, rivers or wind. We then modeled the age structure and relative abundance of the different pools that might have contributed to the older ages of TOC and alkenones. Results suggest the addition of a moderate (15-20%) proportion of very old allochthonous material (18,500-49,900 yr). This information was used to deconvolve the alkenone-derived sea surface temperature (SST) record and assess the impact of allochthonous inputs on paleoclimate signals.

Reinforcing the North Atlantic backbone: revision and extension of the composite splice at ODP Site 982

Abstract. Ocean Drilling Program (ODP) Site 982 represents a key location for understanding the evolution of climate in the North Atlantic over the past 12 Ma. However, concerns exist about the validity and robustness of the underlying stratigraphy and astrochronology, which currently limits the adequacy of this site for high-resolution climate studies. To resolve this uncertainty, we verify and extend the early Pliocene to late Miocene shipboard composite splice at Site 982 using high-resolution XRF core scanning data and establish a robust high-resolution benthic foraminiferal stable isotope stratigraphy and astrochronology between 8.0 and 4.5 Ma. Splice revisions and verifications resulted in ∼ 11 m of gaps in the original Site 982 isotope stratigraphy, which were filled with 263 new isotope analyses. This new stratigraphy reveals previously unseen benthic δ18O excursions, particularly prior to 6.65 Ma. The benthic δ18O record displays distinct, asymmetric cycles between 7.7 and 6.65 Ma, confirming that high-latitude climate is a prevalent forcing during this interval. An intensification of the 41 kyr beat in both the benthic δ13C and δ18O is also observed ∼ 6.4 Ma, marking a strengthening in the cryosphere–carbon cycle coupling. A large ∼ 0.7 ‰ double excursion is revealed ∼ 6.4–6.3 Ma, which also marks the onset of an interval of average higher δ18O and large precession and obliquity-dominated δ18O excursions between 6.4 and 5.4 Ma, coincident with the culmination of the late Miocene cooling. The two largest benthic δ18O excursions ∼ 6.4–6.3 Ma and TG20/22 coincide with the coolest alkenone-derived sea surface temperature (SST) estimates from Site 982, suggesting a strong connection between the late Miocene global cooling, and deep-sea cooling and dynamic ice sheet expansion. The splice revisions and revised astrochronology resolve key stratigraphic issues that have hampered correlation between Site 982, the equatorial Atlantic and the Mediterranean. Comparisons of the revised Site 982 stratigraphy to high-resolution astronomically tuned benthic δ18O stratigraphies from ODP Site 926 (equatorial Atlantic) and Ain el Beida (north-western Morocco) show that prior inconsistencies in short-term excursions are now resolved. The identification of key new cycles at Site 982 further highlights the requirement for the current scheme for late Miocene marine isotope stages to be redefined. Our new integrated deep-sea benthic stable isotope stratigraphy and astrochronology from Site 982 will facilitate future high-resolution late Miocene to early Pliocene climate research.

Heat export from the tropics drives mid to late Holocene palaeoceanographic changes offshore southern Australia

The Leeuwin Current (LC), an eastern boundary current, transports tropical waters from the Indo-Pacific Warm Pool (IPWP) towards southern latitudes and modulates oceanic conditions offshore southern Australia. New, high-resolution planktic foraminifer assemblage data and alkenone-derived sea surface temperatures (SST) provide an in-depth view on LC variability and mechanisms driving the current's properties during the mid to late Holocene (last c. 7.4 ka BP). Our marine reconstructions highlight a longer-term mid to late Holocene reduction of tropical heat export from the IPWP area into the LC. Mid Holocene (c. 7.4 to 3.5 ka BP) occurrence of high SSTs (>19.5 °C), tropical planktic foraminifera and a well-stratified water column document an enhanced heat export from the tropics. From c. 3.5 ka BP onwards, a weaker LC and a notably reduced tropical heat export cause oceanic cooling offshore southern Australia. The observed mid to late Holocene trends likely result from large-scale changes in the IPWP's heat storage linked to the El Niño-Southern Oscillation (ENSO) phenomenon. We propose that a strong and warm LC occurs in response to a La Niña-like state of ENSO during the mid Holocene. The late Holocene LC cooling, however, results from a shift towards an El Niño-like state and a more variable ENSO system that causes cooling of the IPWP. Superimposed on these longer-term trends we find evidence of distinct late Holocene millennial-scale phases of enhanced El Niño/La Niña development, which appear synchronous with northern hemispheric climatic variability. Phases of dominant El Niño-like states occur parallel to North Atlantic cold phases: the ‘2800 years BP cooling event’, the ‘Dark Ages’ and the ‘Little Ice Age’, whereas the ‘Roman Warm Period’ and the ‘Medieval Climate Anomaly’ parallel periods of a predominant La Niña-like state. Our findings provide further evidence of coherent interhemispheric climatic and oceanic conditions during the mid to late Holocene, suggesting ENSO as a potential mediator.

Holocene paleoenvironment changes in the northern Yellow Sea: Evidence from alkenone-derived sea surface temperature

A well-dated, high-resolution sea surface temperature (SST) record based on long chain unsaturated alkenones from a northern Yellow Sea (NYS) sediment core is present in this study. Variations in linear sedimentation rate, alkenone content and SST clearly reveal five distinct stages, consistent with changes in local sea level as well as the evolution of the Yellow Sea Warm Current (YSWC). Before 8.9cal.kyr BP, the marine transgression might not reach to the northern part of the NYS. A shallow coastal environment was formed from 8.9 to 8.5cal.kyr BP, with a subsequent fast transgression during 7.8 to 6.5cal.kyr BP. After 5.8cal.kyr BP, the NYS was dominated by a relatively constant marine environment. SST records reveal that the YSWC was likely to have started since 7.0cal.kyr and strengthened progressively in period of 7.0–5.5cal.kyr BP. After 5.5cal.kyr BP, the SST elevated about 1.5°C, which correlated well to salinity changes along the YSWC indicating that the formation time of the modern YSWC was at about 5.5cal.kyr BP. The SST variation shows a millennial period of ~1500-yr and centennial periods of 131-yr and 113-yr. The ~1.5kyr cycle dominated the period of 8.9–5.5cal.kyr BP, suggesting a tele-connection between the Yellow Sea SST and global climate changes, might through the Kuroshio current. Centennial periods dominated almost all of cold periods recorded in core BY14, implying the signature of solar irradiance cycles by means of the strengthened East Asia Winter Monsoon (EAWM).

Evolution of the southwestern Pacific surface waters during the early Pleistocene

ABSTRACTWe present alkenone-derived sea surface temperature (SST) records from the New Zealand/Tasmania sector of the Southern Ocean (ODP Sites 1171 and 1125) during the Early Pleistocene (c. 1.7 to 0.7 Ma). Our data show that the subantarctic surface waters (ODP Site 1171) cooled by c. 4 °C and reached near modern conditions after 0.88 Ma, indicating a long-term northward migration of the Southern Ocean fronts. This long-term cooling occurred in two major steps between 1.25–1.1 Ma and 0.95–0.88 Ma, which correlate with significant increases in Antarctic ice volume and global circulation changes. The comparison between the subantarctic and subtropical SST trends tends to indicate that the Subtropical Front was unstable during the Early Pleistocene with significant southward incursions of subtropical waters during warm interglacials (1.07 Ma and 0.95 Ma identified as MIS31, MIS25).

The structure of Holocene climate change in mid-latitude North America

A sequence of long-term and rapid changes during the Holocene appears in a network of 40 well-resolved paleoclimate datasets from mid-latitude North America, including records of pollen-inferred temperatures, alkenone-derived sea-surface temperatures (SSTs), lake-level changes, dust accumulation, and lake isotopes from Idaho to Maine. Statistical analyses reveal that changes in insolation and the Laurentide Ice Sheet explain 51.7% of the variance in the records, especially multi-millennial trends, but peak rates of change indicate additional rapid changes at ca. 10.8, 9.4, 8.3, 7.0, 5.5–5.2, 4.7, 2.1, and 0.9 ka. Step changes between 9.4 and 8.3 ka relate to ice sheet dynamics that warmed much of the region, and changes at 5.5 ka were the largest since the demise of the ice sheet. The shift at 5.5 ka initiated widespread cooling and increases in effective moisture, which culminated in the coolest, wettest millennia in most areas after 2.1 ka. Replicated evidence from multiple records also shows a spatially-varied set of multi-century fluctuations including 1) low temperatures and high effective moisture at 5.5–4.8 ka in the mid-continent and 2) repeated phases of low SSTs, cool summers, and drought superimposed upon long cooling, moistening trends in eastern North American since 5.5 ka.

Cooling trend over the past 4 centuries in northeastern Hong Kong waters as revealed by alkenone-derived SST records

The climate history over the past few centuries is important to be used to assess how regional climate responds to global forcing. Here we report three high-resolution alkenone-based sea surface temperature (SST) records over the past 4 centuries from three sediment cores collected in the Mirs Bay, northeastern Hong Kong. All three SST records consistently show a general cooling trend toward the present, with most of cooling occurring over the last century. Alkenone-derived SST values stayed around 26.5–27°C at the three sites prior to 1900s and decreased into the range of 25–26°C. The magnitude of cooling approximately from the Little Ice Age (LIA) to present tends to be dampened from ∼2°C nearshore to ∼1°C offshore. The cooling trend, as identified in all three SST records, is thus opposite to the global temperature rise over the last century. Assisted with modern observations, we interpret that the alkenone-derived SST reflects increasing upwelling in the Mirs Bay, which likely results from the strengthened East Asian summer monsoon, in the context of global warming over the last century.

Contrasting sea surface temperature of summer and winter monsoon variability in the northern Arabian Sea over the last 25 ka

Abstract The seasonal monsoon cycle with winds from the southwest (SW) in summer and from the northeast (NE) in winter strongly impacts on modern regional sea surface temperature (SST) patterns in the Arabian Sea (northern Indian Ocean). To reconstruct the temporal and spatial variation in the dynamically coupled winter and summer monsoon strength over the last 25 ka, we analyzed alkenone-derived SST variations in one sediment core from the northwestern Arabian Sea, that is influenced by the summer monsoon (SST affected by upwelling processes), and in one core from the northeastern Arabian Sea, where SST is mainly governed by the winter monsoon (no upwelling). Comparison of the SST records reveals an antagonistic relationship of summer and winter monsoon strength throughout the late deglaciation and the Holocene. Upwelling along the Arabian Peninsula associated with peak SW monsoonal wind strength was strongest during the early Holocene climate optimum between 11 and 8 ka, and coincided with the northernmost position of the Intertropical Convergence Zone (ITCZ) marked by maximum precipitation over northern Oman. The SW monsoon weakened over the middle to late Holocene, while the NE monsoon gained strength. This different evolution was caused by the southward displacement of the ITCZ throughout the Holocene. Superimposed over the long-term trend are variations in northeast monsoon wind strength at time scales of centuries that were synchronous with late Holocene climate variations recorded on the Asian continent and in the high-latitude Northern Hemisphere. Their likely driving forces are insolation changes associated with sunspot cycles. Enhanced by feedback mechanisms (e.g. land-sea thermal contrast) they enforced centennial scale fluctuations in wind strength and temperature in the northern Arabian Sea monsoon system.

Holocene sea-surface temperature variability in the Chilean fjord region

Here we provide three new Holocene (11–0calka BP) alkenone-derived sea surface temperature (SST) records from the southernmost Chilean fjord region (50–53°S). SST estimates may be biased towards summer temperature in this region, as revealed by a large set of surface sediments. The Holocene records show consistently warmer than present-day SSTs except for the past ~0.6calka BP. However, they do not exhibit an early Holocene temperature optimum as registered further north off Chile and in Antarctica. This may have resulted from a combination of factors including decreased inflow of warmer open marine waters due to lower sea-level stands, enhanced advection of colder and fresher inner fjord waters, and stronger westerly winds. During the mid-Holocene, pronounced short-term variations of up to 2.5°C and a cooling centered at ~5calka BP, which coincides with the first Neoglacial glacier advance in the Southern Andes, are recorded. The latest Holocene is characterized by two pronounced cold events centered at ~0.6 and 0.25calka BP, i.e., during the Little Ice Age. These cold events have lower amplitudes in the offshore records, suggesting an amplification of the SST signal in the inner fjords.

Late Holocene primary productivity and sea surface temperature variations in the northeastern Arabian Sea: Implications for winter monsoon variability

Variability in the oceanic environment of the Arabian Sea region is strongly 34 influenced by the seasonal monsoon cycle of alternating wind directions. Prominent and well studied is the summer monsoon, but much less is known about late Holocene changes in winter monsoon strength with winds from the northeast that drive convective mixing and highsurface ocean productivity in the northeastern Arabian Sea. To establish the first high resolution record of winter monsoon variability for the late Holocene, we analyzed alkenone derived sea surface temperature (SST) variations and proxies of primary productivity (organic carbon and δ15N) in a well-laminated sediment core from the Pakistan continental margin. Increased summer monsoon and weak winter monsoon intensities off Pakistan are indicated from 400 B.C. to 700 A.D. by reduced productivity and relatively high SST. At about 700 A.D. the intensity of the winter monsoon increased off Pakistan as indicated by a trend to lower SST. We infer that winter monsoon was still weak from 700 to 1400 A.D., because primary production did not increase despite decreasing SST. Declining SST and elevated biological production from 1400 to 1900 A.D. suggest invigorated convective winter mixing by strengthening winter monsoon circulation, most likely a regional expression of colder climate conditions during the Little Ice Age on the Northern Hemisphere. The comparison of winter monsoon intensity with records of summer monsoon intensity suggests that an inverse relationship between summer and winter monsoon strength exists in the Asian monsoon system during the late Holocene, effected by shifts in the Intertropical Convergence Zone.

Variations of alkenone temperature in the Sea of Japan during the last 170 ka and its paleoceanographic implications

Two sediment cores, KCES1 and ODP797, which were recovered from the Sea of Japan (JS), were measured for alkenone-derived sea surface temperatures (\(U^{{\text{K}}^{\prime}}_{37} {\text{-SSTs}}\)). Our results revealed that the SSTs closely follow the glacial-interglacial cycles during the last 170 ka, except in the last glacial maximum (LGM), during which the SST was higher than in the Holocene. The anomalous high temperature in the LGM is considered as an effect of the intrusion of a low salinity water mass into the JS when the sea level was almost below 130 m. On the glacial-interglacial to orbital timescale, the \( U^{{\text{K}}^{\prime }}_{37} {\text{-SSTs}} \) record in the JS correlated well with the benthic foraminiferal δ18O record and solar insolation, which suggests the dominant control of solar insolation and its related sea ice development on the SST in the JS. On the sub-orbital/millennial timescale, reduced SST corresponds to an enhanced east asian winter monsoon (EAWM) during the last glacial period (MIS3 and MIS4), indicating the dominant control of sea ice expansion due to the enhanced EAWM on the SST in the JS. In contrast, during the last interglacial period (MIS5), the SST in the JS was controlled by variations in the east Asian summer monsoon. These results highlight the key role of solar insolation and associated glacial-interglacial conditions in the variations of the SST in the JS since the last 170 ka.

Paleoenvironmental conditions for the development of calcareous nannofossil acme during the late Miocene in the eastern equatorial Pacific

Repeated monospecific coccolithophore dominance intervals (acmes) of specimens belonging to the Noelaerhabdaceae family—including the genus Reticulofenestra and modern descendants Emiliania and Gephyrocapsa—occurred during the Neogene. Such acme was recognized during the late Miocene (~ 8.6 Ma), at a time of a major reorganization of nannofossil assemblages resulting in a worldwide temporary disappearance of larger forms of the genus Reticulofenestra (R. pseudoumbilicus) and the gradual recovery and dominance of its smaller forms (< 5 µm). In this study we present a multiproxy investigation of late Miocene sediments from the east equatorial Pacific Integrated Ocean Drilling Program Site U1338 where small reticulofenestrid-type placoliths with a closed central area—known as small Dictyococcites spp. (< 3 µm)—formed an acme. We report on oxygen and carbon stable isotope records of multispecies planktic calcite and alkenone-derived sea surface temperature. Our data indicate that, during this 100 kyr long acme, the east equatorial Pacific thermocline remained deep and stable. Local surface stratification state fails to explain this acme and thus contradicts the model-based hypothesis of a Southern Ocean high-latitude nutrient control of the surface waters in the east equatorial Pacific. Instead, our findings suggest that external forcing such as an extended period of low eccentricity may have created favorable conditions for the small Dictyococcites spp. growth.